CS_BASICS

pic_source

Resource

• LC classics problems

  • Blind Curated 75
  • Grind 75
  • Grind 169
  • LC 官神Github題目分類整理
  • LC top 100 likes
  • neetcode 150 LC list
    • My Brain after 569 Leetcode Problems
  • LC pattern @ blind : Curated-List-of-Top-100-LeetCode-Questions-to-Save-Your-Time
  • LC Algorithm Problem Classification
  • cheatsheet-leetcode-a4
  • 14-patterns-to-ace-any-coding-interview-question
  • grokking-the-coding-interview

• LC experiences

  • LC難度表
    • 數字越高, 題目越難, 挑與自己LC排名接近的題庫
    • (e.g. LC rank ~= 1600, pick 1600 problem)
  • 代碼隨想錄
  • Leetcode cookbook
  • fucking-algorithm
  • fucking-algorithm website
  • FAANG 面試準備經驗與建議（一）
  • FAANG 面試準備經驗與建議（二）
  • LC 小知識
  • Meta SWE isnterview prep
  • 0到100的軟體工程師面試之路
  • 來和大家聊聊我是如何刷題的 : pt1, pt2, pt3
    • https://blog.csdn.net/azl397985856/article/details/110358828
    • https://mp.weixin.qq.com/s/guCR2DCTGoWf4ojeqq2M8A
    • https://mp.weixin.qq.com/s/P_RMRmugmxvIHGyn2EHl7g

• LC Flow

  • high level idea : data structure, algorithm
  • offer time & space complexity
  • code implementation
  • offer test case (consider edge case)
  • discussion & follow up

• Resource.md - Resource for coding interview (keep updating)
• Teach yourself CS
• MindMapCodeInterview - Mind map for coding interview
• CodeInterviewCheatsheet - Coding interview cheetsheet
• repl.it - Coding online!

• Visualization
  • visualgo - DFS / BFS - DFS, BFS visualization
  • visualgo - linkedlist - Linkedlist visualization
  • visualgo - BST - binary search tree visualization
  • toptal-sorting-algorithms- sorting algorithms online

• How to: Work at Google — Example Coding/Engineering Interview
• bit_manipulation.md - Bit Manipulation Cheat Sheet
• Py TimeComplexity - Py basic data structure Time Complexity ref
• Py data model - Python data model doc
• pgexercises - Postgre exercises
• sqlservertutorial
• Books
• freecodecamp - data-structures
• LC interview-experience
• Cheatsheet

• Data structure

  • py core data structure interview

• System design

  • Video
    • Tech Dummies Narendra L Utube
    • nfoQ Utube
  • Blog
    • highscalability.com
    • Le Cloud Blog
    • Irrational Exuberance
    • Crack the system design interview
  • Repo
    • system-design-primer repo

• Tools

  • https://coderpad.io/
  • https://codeshare.io/
  • http://sqlfiddle.com/

• LC SQL resources

  • LC SQL resource

Algorithms content

• Bit Manipulation
• Array
• String
• Linked List
• Stack
• Queue
• Heap
• Tree
• Hash Table
• Math
• Two Pointers
• Sort
• Recursion
• Binary Search
• Binary Search Tree
• Breadth-First Search
• Depth-First Search
• Backtracking
• Dynamic Programming
• Greedy
• Graph
• Geometry
• Simulation
• Design
• Concurrency

Database

• SQL

Shell

• Shell Script

Progress

Data Structure

#	Title	Solution	Use case	Comment	Status
	Linear
	Array	Py			AGAIN*
	Queue	Py, JS			AGAIN*
	Stack	Py, JS (linkedlist), JS (array)			OK
	Hash table	Py, JS	usually for improving time complexity B(O) via extra space complexity (time-space tradeoff)	good basic	AGAIN****
	Linear, Pointer
	LinkedList	Py, JS, Java			OK**
	Doubly LinkedList	Python, JS			AGAIN
	Non Linear, Pointer
	Tree	Py			AGAIN**
	Binary search Tree (BST)	Python, JS, Scala, Java			AGAIN
	Binary Tree	Py			AGAIN**
	Trie	Py			AGAIN
	Heap	heap.py, MinHeap.py, MaxHeap.py, MinHeap.java, MaxHeap.java			AGAIN
	Priority Queue (PQ)	Py 1, Py 2, Py 3			AGAIN
	Graph
	Graph	Py, JS. Java1, Java2			OK***
	DirectedEdge	Java			AGAIN

Algorithm

#	Title	Solution	Use case	Comment	Time complexity	Space complexity	Status
	Binary search	Python	complexity ref	Best : O(1), Avg : O(log N), Worst : O(log N)			AGAIN
	Linear search	Python					AGAIN
	Breadth-first search (BFS)	Python, JS	FIND SHORTEST PATH				AGAIN***
	Depth-first search (DFS)	Python, JS	TO CHECK IF SOMETHING EXIST	inorder, postorder, postorder (can recreate a tree)			AGAIN***
	Bubble sort	Python, JS , C					OK* (3)
	Insertion sort	Python, JS	stable sort	work very fast for nearly sorted array	Best :O(n), Average : O(n^2), Worst : O(n^2)	Worst : O(1)	AGAIN
	Bucket sort	Python					AGAIN
	Quick sort	quick_sort.py, quick_sort_v2.py, QuickSort.java	NOTE !!!, avg is NLogN, worst is N**2, big O ref, big O ref 2	Best : O(N Log N), Avg : O(N Log N), Worst : O(N^2)		AGAIN***
	Heap sort	Python	big O ref	Best : O(N Log N), Avg : O(N Log N), Worst : O(N Log N)		AGAIN**
	Merge sort	merge_sort_topdown.py, mergesort_bottomup.py, MergeSortTopDown.java, SQL			Best : O(N Log N), Avg : O(N Log N), Worst : O(N Log N)	O(N)	OK* (2)
	Pancake sort	Python					AGAIN
	Selection sort	Python, JS					AGAIN
	Topological sort	Python, Java		Topological Sort is a algorithm can find "ordering" on an "order dependency" graph			AGAIN
	md5	Python					AGAIN
	Union Find	Python 1, Python 2, Java 1			AGAIN
	Dynamic programming	JS 1, fibonacci_dp JS					AGAIN
	Dijkstra	Python,Java			AGAIN***
	Floyd-Warshall	Python					not start
	Bellman-Ford	Python					not start
	Quick Find	Python, Java	init : O(N), union : O(N), find : O(1)	simple, but slow			AGAIN
	Quick Union	Java, Java v2	init : O(N), union : O(N), find : O(N)	lazy approach, route compression, optimized Quick Find			AGAIN
	Quick Union (Improvements)		lazy approach, path compression			AGAIN
	Priority Queue (unsorted)	Java				AGAIN
	LRU cache	Python		LC 146			AGAIN
	LFU Cache	Python		LC 460			AGAIN

Array

#	Title	Solution	Time	Space	Difficulty	Note	Status
015	3 Sum	Python, Java	O(n^2)	O(1)	Medium	Curated Top 75, good basic, check with # 001 Two Sum, #018 4 sum, Two Pointers, good basic,amazon,fb	OK******* (7)
016	3 Sum Closest	Python	O(n^2)	O(1)	Medium	Two Pointers, basic, good trick,3 sum,google, fb, apple, m$, GS, amazon	AGAIN**** (2)
018	4 Sum	Python	O(n^3)	O(1)	Medium	k sum, Two Pointers, check #016 3 sum, # 454 4 SUM II,good trick, fb	AGAIN******** (4)
026	Remove Duplicates from Sorted Array	Python, Scala, Java	O(n)	O(1)	Easy	Two Pointers, basic, good trick,M$ ,fb	AGAIN************** (11)
027	Remove Element	Python, Scala	O(n)	O(1)	Easy	basic, amazon	OK* (2)
031	Next Permutation	Python, Scala, Java	O(n)	O(1)	Medium	good trick, check, 2 pointers, Uber, GS, google,amazon, fb	AGAIN****************** (8) (MUST)
041	First Missing Positive	Python	O(n)	O(1)	Hard	good trick, hash key , array, LC top 100 like, amazon, fb, google, apple, uber	AGAIN******** (2)
048	Rotate Image	Python, Java	O(n^2)	O(1)	Medium	Curated Top 75, basic, i,j->j,i transpose matrix, garena, amazon, apple	AGAIN************* (5) (MUST)
054	Spiral Matrix	Python, Java	O(m * n)	O(1)	Medium	Curated Top 75, good basic, array boundary, matrix,amazon	AGAIN**** (4)
057	Insert Interval	Python, Java			Medium	Curated Top 75, good basic, 056 Merge Intervals, java array, list basic	AGAIN********** (5) (MUST!)
059	Spiral Matrix II	Python	O(n^2)	O(1)	Medium	check # 054 Spiral Matrix, amazon	AGAIN** (2)
066	Plus One	Python, Java	O(n)	O(1)	Easy	basic, digit, google	AGAIN** (3)
073	Set Matrix Zeroes	Python, Java	O(m * n)	O(1)	Medium	Curated Top 75, matrix, amazon	OK* (4)
080	Remove Duplicates from Sorted Array II	Python, Java	O(n)	O(1)	Medium	two pointers,AGAIN, basic, trick, fb, #26 Remove Duplicates from Sorted Array	AGAIN************ (8)
118	Pascal's Triangle	Python	O(n^2)	O(1)	Easy	trick, basic	AGAIN** (2)
119	Pascal's Triangle II	Python	O(n^2)	O(1)	Easy	array, check with # 118 Pascal's Triangle, amazon	OK** (4)
121	Best Time to Buy and Sell Stock	Python, Java	O(n)	O(1)	Easy	Curated Top 75, dp,basic,greedy,UBER, M$, amazon, fb	OK****** (7) (but again)
157	Read N Characters Given Read4	Python	O(n)	O(1)	Easy	🔒, buffer, google, amazon, fb	AGAIN**** (3)
163	Missing Ranges	Python, Java	O(n)	O(1)	Medium	two pointer, 🔒, google, amazon	OK******* (4)
169	Majority Element	Python, Java	O(n)	O(1)	Easy	amazon	OK*
189	Rotate Array	Python	O(n)	O(1)	Medium	array, k % len(nums), good basic, amazon	OK* (7) (but again)
209	Minimum Size Subarray Sum	Python	O(n)	O(1)	Medium	good basic, sliding window, Binary Search, trick,2 pointers, fb	OK********** (5) (but again)
215	Kth Largest Element in an Array	Python, Java	O(n) ~ O(n^2)	O(1)	Medium	EPI, quick sort, bubble sort, sort, amazon,fb	OK* (2)
228	Summary Ranges	Python, Java	O(n)	O(1)	Easy	check # 163 Missing Ranges, basic, array, google	OK*** (4)
229	Majority Element II	Python	O(n)	O(1)	Medium		OK
238	Product of Array Except Self	Python, Java	O(n)	O(1)	Medium	Curated Top 75, left, right array, LintCode, good trick, Apple, M$, amazon, fb	OK******* (but again)(6)
240	Search a 2D Matrix II	Python	O(m + n)	O(1)	Medium	LintCode, basic, matrix, trick, binary search, check # 74 Search a 2D Matrix, amazon, apple	OK***** (7) (but again)
243	Shortest Word Distance	Python, Java	O(n)	O(1)	Easy	🔒, basic	OK*
245	Shortest Word Distance III	Python	O(n)	O(1)	Medium	🔒, check Shortest Word Distance I II III	OK*
251	Flatten 2D Vector	Python	O(1)	O(1)	Medium	🔒, good basic, check# 341 Flatten Nested List Iterator, google, airbnb, twitter, amazon	AGAIN******** (4)
277	Find the Celebrity	Python	O(n)	O(1)	Medium	graph, 🔒, fb,amazon, trick	AGAIN********** (5)
287	Find the Duplicate Number	Python			Medium	good trick,binary search, cycle linked list, check # 142 Linked List Cycle II, amazon	OK* (2)
289	Game of Life	Python	O(m * n)	O(1)	Medium	simultaneous,matrix,complex, google, amazon	AGAIN*** (4)
293	Flip Game	Python	O(n * (c+1))	O(1)	Easy	🔒, basic	AGAIN*
308	Range Sum Query 2D - Mutable	Python	O(n * (c+1))	O(1)	Hard	LC 303, pre-sum, matrix, tree, 🔒, google	not start
311	Sparse Matrix Multiplication	Python	O(m * n * l)	O(m * l)	Medium	🔒, matrix, basic, fb	OK**** (4)
334	Increasing Triplet Subsequence	Python, Java	O(n)	O(1)	Medium	AGAIN, check # 300 Longest Increasing Subsequence, trick, fb, google	OK***** (but again) (4)
370	Range Addition	Python	O(k + n)	O(1)	Medium	prefix sum, array, 🔒, check # 598 Range Addition II, amazon	AGAIN************** (4) (MUST)
384	Shuffle an Array	Python	O(n)	O(n)	Medium	EPI, trick	OK*
396	Rotate Function	Python	O(n)	O(1)	Medium	can pass, math, trick, amazon	AGAIN*** (3)
412	Fizz Buzz	Python, Scala	O(n)	O(1)	Easy		OK
414	Third Maximum Number	Python	O(n)	O(1)	Easy	good basic, amazon	OK* (4)
419	Battleships in a Board	Python	O(m * n)	O(1)	Medium		AGAIN*
422	Valid Word Square	Python	O(m * n)	O(1)	Easy	🔒, basic, matrix	AGAIN*
442	Find All Duplicates in an Array	Python	O(n)	O(1)	Medium		OK*
448	Find All Numbers Disappeared in an Array	Python	O(n)	O(1)	Easy		OK
498	Diagonal Traverse	Python	O(n)	O(1)	Medium	matrix,google,fb	AGAIN*** (3) (not start)
531	Lonely Pixel I	Python	O(m * n)	O(m + n)	Medium	🔒, matrix, basic	AGAIN*
533	Lonely Pixel II	Python	O(m * n)	O(m * n)	Medium	🔒	AGAIN (not start*)
565	Array Nesting	Python	O(n)	O(1)	Medium	union find,dfs,apple	AGAIN**** (1)
566	Reshape the Matrix	Python	O(m * n)	O(m * n)	Easy	basic, matrix	AGAIN**
581	Shortest Unsorted Continuous Subarray	Python	O(n)	O(1)	Easy	basic	AGAIN*
605	Can Place Flowers	Python	O(n)	O(1)	Easy	pass vs continue	OK*
624	Maximum Distance in Arrays	Python	O(n)	O(1)	Easy	🔒	AGAIN*
643	Maximum Average Subarray I	Python	O(n)	O(1)	Easy	Math, basic	AGAIN*
661	Image Smoother	Python	O(m * n)	O(1)	Easy	matrix,basic, amazon	OK***** (4) (but again!)
665	Non-decreasing Array	Python	O(n)	O(1)	Easy		AGAIN (not start)
667	Beautiful Arrangement II	Python	O(n)	O(1)	Medium		AGAIN (not start)
670	Maximum Swap	Python	O(logn)	O(logn)	Medium	good basic, reversed, str -> list,string + pointers , fb	AGAIN************ (9) (MUST)
674	Longest Continuous Increasing Subsequence	Python, Java	O(n)	O(1)	Easy	good basic,dp,2 pointers ,array ,fb	OK*** (5)
697	Degree of an Array	Python	O(n)	O(n)	Easy		AGAIN (not start)
713	Subarray Product Less Than K	Python	O(n)	O(1)	Medium	basic, sliding window, good trick	OK****** (3) (but again)
717	1-bit and 2-bit Characters	Python	O(n)	O(1)	Easy	Greedy	AGAIN (not start)
723	Candy Crush	Python	O((R * C)^2)	O(1)	Medium	complex	AGAIN (not start)
724	Find Pivot Index	Python	O(n)	O(1)	Easy		OK*
729	My Calendar I	Python, Java	O(nlogn)	O(n)	Medium	good basic, brute force, binary search, google	AGAIN**** (1)
731	My Calendar II	Python	O(n^2)	O(n)	Medium	trick, basic	AGAIN** (not start)
747	Largest Number At Least Twice of Others	Python	O(n)	O(1)	Easy	good basic, data structure	OK*
755	Pour Water	Python	O(v * n)	O(1)	Medium	complex	AGAIN (not start)
766	Toeplitz Matrix	Python	O(m * n)	O(1)	Easy	basic, matrix, google	AGAIN* (2)
769	Max Chunks To Make Sorted	Python	O(n)	O(1)	Medium		AGAIN** (not start)
792	Number of Matching Subsequences	Python, Java	O(n + w)	O(1)	Medium	basic, hash map, google	AGAIN**** (2)
794	Valid Tic-Tac-Toe State	Python	O(1)	O(1)	Medium	complex	AGAIN
795	Number of Subarrays with Bounded Maximum	Python	O(n)	O(1)	Medium		AGAIN (not start*)
807	Max Increase to Keep City Skyline	Python	O(n^2)	O(n)	Medium		AGAIN*
821	Shortest Distance to a Character	Python	O(n)	O(1)	Easy	basic, trick	AGAIN**
830	Positions of Large Groups	Python	O(n)	O(1)	Easy	basic	AGAIN*
832	Flipping an Image	Python	O(n^2)	O(1)	Easy		AGAIN*
835	Image Overlap	Python	O(n^4)	O(n^2)	Medium		AGAIN (not start)
840	Magic Squares In Grid	Python	O(m * n)	O(1)	Easy	complex	AGAIN (not start)
842	Split Array into Fibonacci Sequence	Python	O(n^3)	O(n)	Medium	check # 306 Addictive Number, basic, dfs, fibonacci	AGAIN* (not start)
845	Longest Mountain in Array	Python	O(n)	O(1)	Medium	basic, trick	AGAIN* (not start)
849	Maximize Distance to Closest Person	Python	O(n)	O(1)	Easy	basic, trick	AGAIN (not start*)
860	Lemonade Change	Python	O(n)	O(1)	Easy	amazon	OK* (2)
0868	Transpose Matrix	Python	O(r * c)	O(1)	Easy	basic	OK*
885	Spiral Matrix III	Python	O(max(m, n)^2)	O(1)	Medium	basic	AGAIN* (not start)
888	Fair Candy Swap	Python	O(m + n)	O(m + n)	Easy	basic, trick	OK*
896	Monotonic Array	Python	O(n)	O(1)	Easy	all,array ,fb	OK
905	Sort Array By Parity	Python	O(n)	O(1)	Easy	sort by lambda	OK
909	Snakes and Ladders	Python	O(n^2)	O(n^2)	Medium	array, matrix, dfs,complex, amazon	AGAIN***** (3)
915	Partition Array into Disjoint Intervals	Python	O(n)	O(n)	Medium	basic, trick	AGAIN* (not start)
918	Maximum Sum Circular Subarray	Python	O(n)	O(1)	Medium	check # 053 Maximum Subarray, basic, trick	AGAIN* (not start)
921	Minimum Add to Make Parentheses Valid	Python	O(n)	O(1)	Medium	basic, trick	AGAIN**
922	Sort Array By Parity II	Python	O(n)	O(1)	Easy	basic	AGAIN*
923	3Sum With Multiplicity	Python	O(n^2)	O(n)	Medium		AGAIN (not start)
932	Beautiful Array	Python, Java	O(n)	O(n)	Medium	basic, trick, google	AGAIN** (not start) (1)
941	Valid Mountain Array	Python	O(n)	O(1)	Easy	basic, good basic	AGAIN*
945	Minimum Increment to Make Array Unique	Python	O(nlogn)	O(n)	Medium	trick, good	AGAIN**
947	Most Stones Removed with Same Row or Column	Python, Java	O(n)	O(n)	Medium	Union Find , dfs, trick,google	AGAIN** (1) (not start)
949	Largest Time for Given Digits	Python	O(1)	O(1)	Easy		OK*
950	Reveal Cards In Increasing Order	Python	O(n)	O(n)	Medium		AGAIN (not start)
954	Array of Doubled Pairs	Python	O(n + klogk)	O(k)	Medium	counter, dict, trick	AGAIN**
961	N-Repeated Element in Size 2N Array	Python	O(n)	O(1)	Easy		OK
989	Add to Array-Form of Integer	Python	O(n)	O(1)	Easy	add xxx to sum, fb	AGAIN (not start)
004	Median of Two Sorted Arrays	Python	O(n)	O(1)	Hard	array, heapq, binary search, amazon	AGAIN* (1)
010	Regular Expression Matching	Python	O(n)	O(1)	Hard	recursion, dp, fb	AGAIN (not start)
056	Merge Intervals	Python, Java	O(n)	O(1)	Medium	Curated Top 75, good trick,057 Insert Interval, twitter,M$,UBER,google,amazon,fb	OK********* (6) (but AGAIN)
759	Employee Free Time	Python	O(n)	O(1)	Hard	LC 56 and LC 986, heap, merger intervals, scanning line, amazon, fb, google, uber, airbnb	AGAIN********** (2) (not start)
1007	Minimum Domino Rotations For Equal Row	Java	O(n)	O(1)	Medium	google, array, dp	AGAIN (1)
1014	Best Sightseeing Pair	Python			Medium	dp, array, good basic, Spotify	AGAIN (not start)
1027	Longest Arithmetic Subsequence	Python			Medium	dp, hash table, trick, google, amazon	AGAIN*** (not start)
1041	Robot Bounded In Circle	Python			Medium	array, math, amazon	AGAIN** (2)
1109	Corporate Flight Bookings	Python			Medium	array, amazon	AGAIN (not start)
1248	Count Number of Nice Subarrays	Python			Medium	LC 828, hash map, Prefix sum, dict, windown, good basic, deque, array, amazon	AGAIN************** (3) (MUST)
1275	Find Winner on a Tic Tac Toe Game	Python			Easy	amazon	AGAIN (not start)
1567	Maximum Length of Subarray With Positive Product	Python			Medium	good trick, array, dp, 2 pointers, amazon	AGAIN**** (1) (not start)
1914	Cyclically Rotating a Grid	Python			Medium	array, brute force, dequeueamazon	AGAIN (not start)
3195	Find the Minimum Area to Cover All Ones I	Java			Medium	array, LC weekly	AGAIN (1)

Set

#	Title	Solution	Time	Space	Difficulty	Note	Status
2357	Make Array Zero by Subtracting Equal Amounts	Java	O(n^2)	O(1)	Medium	set	Again (1)

Hash Table

#	Title	Solution	Time	Space	Difficulty	Status	Note
001	Two Sum	Python, Java,Scala	O(n)	O(n)	Easy	good basic, hash table, apple, amazon, UBER, grab, fb	OK*** (6) (but again)
003	Longest Substring Without Repeating Characters	Python, Java	O(n)	O(1)	Medium	Curated Top 75, 2 pointers, optimized (SLIDING WINDOW + DICT),good trick, hashMap, MUST, apple,amazon,fb	AGAIN**************** (11) (but AGAIN)
036	Valid Sudoku	Python, Java	O(9^2)	O(9)	Medium	array, hashset, UBER, apple, amazon	OK**** (3) (again)
049	Group Anagrams	Python, Scala, Java	O(n * glogg)	O(n)	Medium	Curated Top 75, good basic, dict,trick,dict + sort, UBER, amazon, fb	OK ********** (6)
170	Two Sum III - Data structure design	Python	O(n)	O(n)	Easy	🔒, trick,good basic, linkedin, fb	OK*** (4)
187	Repeated DNA Sequences	Python	O(n)	O(n)	Medium	basic,trick,set, linkedin	AGAIN**
202	Happy Number	Python , Java	O(k)	O(k)	Easy	twitter,airbnb, UBER	AGAIN** (4)
204	Count Primes	Python	O(n)	O(n)	Medium	good trick, cache, dict, set, M$, amazon	AGAIN******** (5)
205	Isomorphic Strings	Python, Java	O(n)	O(1)	Easy	basic, trick, bloomberg	AGAIN**
217	Contains Duplicate	Python , Java	O(n)	O(n)	Easy	Curated Top 75, yahoo, airbnb	OK
219	Contains Duplicate II	Python	O(n)	O(n)	Easy	basic, trick, hash table	AGAIN*
244	Shortest Word Distance II	Python	ctor: O(n), lookup: O(a + b)	O(n)	Medium	🔒	AGAIN (not start*)
246	Strobogrammatic Number	Python, Java	O(n)	O(1)	Easy	🔒, google, fb	OK* (5)
249	Group Shifted Strings	Python , Java	O(nlogn)	O(n)	Medium	LC 49, 🔒, apple, UBER, google	AGAIN****(5)
266	Palindrome Permutation	Python	O(n)	O(1)	Easy	🔒	OK
288	Unique Word Abbreviation	Python	ctor: O(n), lookup: O(1)	O(k)	Medium	🔒, ?, google	AGAIN (2)(not start)
290	Word Pattern	Python	O(n)	O(c)	Easy	basic, chcek # 205 Isomorphic Strings,good basic, for ptn, word in zip(pattern, words), dropbox , UBER	AGAIN* (3)
299	Bulls and Cows	Python	O(n)	O(1)	Easy	trick,map(operator.eq, a, b),amazon, airbnb, google	AGAIN* (3)
314	Binary Tree Vertical Order Traversal	Python	O(n)	O(n)	Medium	🔒, BFS,DFS,hash table, tree, trick,google, amazon,fb, m$	OK**** (6)
325	Maximum Size Subarray Sum Equals k	Python, Java	O(n)	O(n)	Medium	prefix sum, 🔒, basic, good trick, dict, fb	AGAIN************** (8) (MUST)
356	Line Reflection	Python	O(n)	O(n)	Medium	🔒, Hash, Two Pointers,math,google	AGAIN (not start)
359	Logger Rate Limiter	Python, Java	O(1), amortized	O(k)	Easy	🔒, google	OK
387	First Unique Character in a String	Python	O(n)	O(n)	Easy	amazon, apple, fb	OK
388	Longest Absolute File Path	Python	O(n)	O(d)	Medium	stack, hash table, good trick, file system, google	AGAIN******* (3)
409	Longest Palindrome	Python	O(n)	O(1)	Easy		OK*
424	Longest Repeating Character Replacement	Python, Java	O(n)	O(1)	Medium	Curated Top 75, silding window, two pointers, good trick, LC blind pattern	AGAIN************* (8) (MUST)
438	Find All Anagrams in a String	Python	O(n)	O(1)	Medium	trick, AGAIN, sliding window,acc Counter,amazon,fb, apple, google, uber, yahoo	AGAIN************** (8)
447	Number of Boomerangs	Python	O(n^2)	O(n)	Easy	trick, google	AGAIN (3) (not start)
454	4Sum II	Python	O(n^2)	O(n^2)	Medium	check LC 018 4SUM, trick, basic, amazon	AGAIN**
463	Island Perimeter	Python	O(n)	O(k)	Medium	basic,hash table, google, fb, amazon	AGAIN** (2)
470	Implement Rand10() Using Rand7()	Python	O(1)	O(1)	Medium	trick, google	AGAIN** (3)
473	Matchsticks to Square	Python	O(n * s * 2^n)	O(n * (2^n + s))	Medium	complex,hard	AGAIN (not start)
523	Continuous Subarray Sum	Python	O(n)	O(k)	Medium	sub array sum, check # 560 Subarray Sum Equals K,good trick,substring, hash table,AGAIN,M$,fb, apple	AGAIN*************** (10) (MUST)
525	Contiguous Array	Python	O(n)	O(n)	Medium	sub-array sum, good basic, array, hashmap, cache, AGAIN, fb, amazon	AGAIN*************** (11) (MUST)
532	K-diff Pairs in an Array	Python	O(n)	O(n)	Medium	hash table, basic, collections.Counter(),a-b =k -> a = k + b , amazon	AGAIN********** (6)
554	Brick Wall	Python	O(n)	O(m)	Medium	trick,hash map, bloomberg, fb, grab	AGAIN******** (5)
560	Subarray Sum Equals K	Python	O(n)	O(n)	Medium	must check,check # 523 Continuous Subarray Sum, LC 1268,basic, substring,good trick,google,fb	AGAIN************** (6) (MUST)
561	Array Partition I	Python	O(r)	O(r)	Easy		OK
575	Distribute Candies	Python	O(n)	O(n)	Easy		OK
594	Longest Harmonious Subsequence	Python	O(n)	O(n)	Easy	basic, good trick	OK* (3)
599	Minimum Index Sum of Two Lists	Python	O((m + n) * l)	O(m * l)	Easy	yelp	OK*
609	Find Duplicate File in System	Python	O(n * l)	O(n * l)	Medium	collections.defaultdict(list), dropbox	OK*
657	Robot Return to Origin	Python			Easy	amazon	OK (2)
748	Shortest Completing Word	Python	O(n)	O(1)	Easy	collections.Counter, hash table,google	AGAIN* (3)
760	Find Anagram Mappings	Python	O(n)	O(n)	Easy	basic, collections.defaultdict	OK
771	Jewels and Stones	Python	O(m + n)	O(n)	Easy	amazon	OK (2)
811	Subdomain Visit Count	Python	O(n)	O(n)	Easy	indeed, bloomberg	AGAIN (not start)
822	Card Flipping Game	Python	O(n)	O(n)	Medium	good basic	AGAIN* (3)
825	Friends Of Appropriate Ages	Python	O(a^2 + n)	O(a)	Medium	good basic,counter,AGAIN,fb	AGAIN***** (5)
869	Reordered Power of 2	Python	O(1)	O(1)	Medium	trick,basic, bit manipulation, bit	AGAIN*
873	Length of Longest Fibonacci Subsequence	Python	O(n^2)	O(n)	Medium	trick, DP, Fibonacci, set	AGAIN**
957	Prison Cells After N Days	Python	O(1)	O(1)	Medium	trick, DP, mod	AGAIN (not start)
966	Vowel Spellchecker	Python	O(n)	O(w)	Medium	trick, dict, set	AGAIN (not start)
974	Subarray Sums Divisible by K	Python	O(n)	O(k)	Medium	variant of # 560 Subarray Sum Equals K , trick, basic	AGAIN** (3) (not start)
992	Subarrays with K Different Integers	Python	O(n)	O(k)	Hard	2 pointers, hashmap, sliding window, amazon	AGAIN (not start)
1010	Pairs of Songs With Total Durations Divisible by 60	Python			Medium	good basicm dict, array, amazon	AGAIN********* (4) (MUST)
1099	Two Sum Less Than K	Python			Medium	dict, sort, amazon	AGAIN* (1) (not start)
1131	Rank Transform of an Array	Python			Easy	dict, array, amazon	OK* (1)
1170	Compare Strings by Frequency of the Smallest Character	Java			Medium	hashmap, binary search, google	AGAIN*** (1)
1296	Divide Array in Sets of K Consecutive Numbers	Python			Medium	LC 846, dict, google	AGAIN (not start)
1923	Longest Common Subpath	Python	O(n)	O(h)	Hard	hash, bit, amazon	AGAIN (not start)

Linked list

#	Title	Solution	Time	Space	Difficulty	Note	Status
002	Add Two Numbers	Python, Java	O(n)	O(1)	Medium	Curated Top 75, check # 445 Add Two Numbers II, basic, linked list, airbnb, amazon, fb	OK**** (6) (but again)
021	Merge Two Sorted Lists	Python, Java	O(n)	O(1)	Easy	Curated Top 75, UBER, amazon, apple, fb	OK******** (6) (but again)
023	Merge k sorted lists	Python, Java	O(n)	O(h)	Hard	Curated Top 75, linked list, check #21 Merge Two Sorted Lists, amazon	OK**** (2) (but again !!!
024	Swap Nodes in Pairs	Python, Java	O(n)	O(1)	Medium	GOOD basic, LC 100 like, UBER,amazon, fb	AGAIN**************** (7) (MUST)
025	Reverse Nodes in k-Group	Python, Java	O(n)	O(1)	Hard	good trick, reverse linkedlist, reverse K linkedlist, amazon, google, fb, m$	AGAIN********** (4)
061	Rotate List	Python	O(n)	O(1)	Medium	basic	AGAIN (2)
082	Remove Duplicates from Sorted List II	Python	O(n)	O(1)	Medium	check # 083 Remove Duplicates from Sorted List	AGAIN (2)
083	Remove Duplicates from Sorted List	Python, Java	O(n)	O(1)	Easy	basic	OK*
092	Reverse Linked List II	Python	O(n)	O(1)	Medium	# 206 Reverse Linked List, good trick, fb, apple, google, amazon, m$	AGAIN*********** (5 (MUST)
138	Copy List with Random Pointer	Python, Java	O(n)	O(1)	Medium	trick, recursive, linked list,hash table, UBER, M$, amazon, fb	AGAIN******* (7)
160	Intersection of Two Linked Lists	Python, Java	O(m + n)	O(1)	Easy	basic, hash table, 2 pointers, linkedlist, airbnb, amazon,fb	OK******* (7) (again!)
203	Remove Linked List Elements	Python , Java	O(n)	O(1)	Easy	linked list basic,amazon	OK** (3)
206	Reverse Linked List	Python, Java	O(n)	O(1)	Easy	Curated Top 75, good basic,amazon,fb	OK*********** (9) (MUST again)
234	Palindrome Linked List	Python, Java	O(n)	O(1)	Easy	linked list,amazon,fb	OK (4)
237	Delete Node in a Linked List	Python	O(1)	O(1)	Easy	LintCode, apple	OK * (1) (but again)
328	Odd Even Linked List	Python	O(n)	O(1)	Medium	basic	OK** (2)
369	Plus One Linked List	Python	O(n)	O(1)	Medium	🔒, basic, trick	AGAIN* (2)
445	Add Two Numbers II	Python	O(m + n)	O(m + n)	Medium	trick, linked list, string,good basic, amazon	AGAIN*** (3)
725	Split Linked List in Parts	Python	O(n + k)	O(1)	Medium	mod, split linked list, linked list, good trick,amazon	AGAIN************ (6) (again)
817	Linked List Components	Python	O(m + n)	O(m)	Medium		OK*
430	Flatten a Multilevel Doubly Linked List	Python, Java			Medium	good trick,doubly linked list,AGAIN,dfs, fb, google	AGAIN********* (4)
707	Design Linked List	Python, Java	O(n)	O(h)	Medium	linked list basic OP	AGAIN (1)
708	Insert into a Cyclic Sorted List	Python	O(n)	O(h)	Medium	AGAIN,cyclic linked list,good trick, google, amazon, fb	AGAIN******** (4)

Stack

#	Title	Solution	Time	Space	Difficulty	Note	Status
020	Valid Parentheses	Python, Java	O(n)	O(n)	Easy	Curated Top 75, good basic,fb, amazon	OK** (6)
032	Longest Valid Parentheses	Python	O(n)	O(n)	Hard	brute force, dp, stackm deque, LC top 100 likes, amazon, fb, m$	AGAIN***** (1) (not start)
071	Simplify Path	Python	O(n)	O(n)	Medium	basic, stack, amazon,fb	OK** (5)
085	Maximal Rectangle	Python	O(n)	O(n)	Hard	top-100-like, brute force, stack, dp, LC 084, google, amazon, apple	AGAIN (not start)
101	Symmetric Tree	Python, Java	O(n)	O(h)	Easy	good basic,bfs, dfs, linkedin, M$,amazon, fb, google	AGAIN********** (5)
150	Evaluate Reverse Polish Notation	Python, Java	O(n)	O(n)	Medium	stack, good trick,amazon	AGAIN********* (5)
155	Min Stack	Python, Java	O(n)	O(1)	Medium	basic, stack, data structure, amazon	OK******* (5)
173	Binary Search Tree Iterator	Python	O(1)	O(h)	Medium	good basic,tree, stack, M$, linkedin, google,amazon,fb	OK***** (5)
224	Basic Calculator	Python	O(n)	O(n)	Hard	basic,trick,stack, LC 227, amazon	AGAIN******* (3)
227	Basic Calculator II	Python	O(n)	O(n)	Medium	delay op, LC 224, good trick,stack,airbnb ,fb, amazon	AGAIN************** (6) (MUST)
232	Implement Queue using Stacks	Python	O(1), amortized	O(n)	Easy	stack, stack-queue, EPI, LintCode, amazon	OK** (2)
255	Verify Preorder Sequence in Binary Search Tree	Python	O(n)	O(1)	Medium	🔒	AGAIN (not start)
331	Verify Preorder Serialization of a Binary Tree	Python	O(n)	O(1)	Medium		AGAIN (not start)
341	Flatten Nested List Iterator	Python	O(n)	O(h)	Medium	LC 284, 🔒 Iterator, stack, generator, good basic,amazon,fb***	AGAIN********* (6)
385	Mini Parser	Python	O(n)	O(h)	Medium		AGAIN
394	Decode String	Python	O(n)	O(h)	Medium	stack, good basic!!!!, pre num string, LC 224, LC 227, amazon, google	AGAIN************** (6) (MUST)
439	Ternary Expression Parser	Python	O(n)	O(1)	Medium	🔒	AGAIN (not start)
456	132 Pattern	Python	O(n)	O(n)	Medium		AGAIN (not start)
496	Next Greater Element I	Python, Java	O(n)	O(n)	Easy	good basic, stack,monotonic stack, LC 739, LC 503, LC 406	OK********* (5) (but again)
503	Next Greater Element II	Python	O(n)	O(n)	Medium	good basic, stack, LC 739, LC 503, LC 406	AGAIN************* (7)
636	Exclusive Time of Functions	Python	O(n)	O(n)	Medium	trick,AGAIN,stack,UBER, fb	AGAIN********** (4)
682	Baseball Game	Python, Java	O(n)	O(n)	Easy	good basic, stack,amazon	OK* (3)
735	Asteroid Collision	Python	O(n)	O(n)	Medium	good basic, stack, m$, GS, fb, amazon	AGAIN************** (5)
739	Daily Temperatures	Python, Java	O(n)	O(n)	Medium	LC 739, LC 503, LC 406, LC 496, LC 42, Monotonic stack, good trick,amazon	AGAIN******************* (12) (MUST)
772	Basic Calculator III	Python	O(n)	O(n)	Medium	check LC 224, 227	AGAIN (not start)
853	Car Fleet	Python, Java	O(nlogn)	O(n)	Medium	array, stack, good trick, google, amz	AGAIN***** (2)
856	Score of Parentheses	Python	O(n)	O(1)	Medium		AGAIN (not start)
872	Leaf-Similar Trees	Python, Java	O(n)	O(h)	Easy		AGAIN*
895	Maximum Frequency Stack	Python	O(n)	O(h)	Hard	good basic, heap, design, stack, amazon, apple, linkedin, m$, twitter	AGAIN********** (1) (not start)
901	Online Stock Span	Python	O(n)	O(n)	Medium		AGAIN (not start)
946	Validate Stack Sequences	Python	O(n)	O(n)	Medium		AGAIN
1047	Remove All Adjacent Duplicates in String	Python, Java	O(n)	O(n)	easy	good basic, fb, amazon, google	AGAIN************ (3) (MUST)
1209	Remove All Adjacent Duplicates in String II	Python	O(n)	O(n)	Medium	good basic, stack, two pointers, greedy, fb, amamzon, apple, spotify	AGAIN************ (3) (MUST)
1703	Minimum Adjacent Swaps for K Consecutive Ones	Python	O(n)	O(n)	Hard	heap, sliding window, amazon	AGAIN (not start)
1896	Minimum Cost to Change the Final Value of Expression	Python	O(n)	O(n)	Hard	complex, stack, dp, dp+stack, dfs, amazon	AGAIN (not start)

Tree

#	Title	Solution	Time	Space	Difficulty	Note	Status
094	Binary Tree Inorder Traversal	Python	O(n)	O(1)	Medium	tree, iteration, recursion, good basic,Morris Traversal, M$, fb	AGAIN********* (4) (MUST)
124	Binary Tree Maximum Path Sum	Python, Java	O(n)	O(1)	Hard	Curated Top 75, google, amazon, fb, m$, twitter	AGAIN******* (2)
144	Binary Tree Preorder Traversal	Python	O(n)	O(1)	Medium	Morris Traversal	AGAIN
208	Implement Trie (Prefix Tree)	Python, Java	O(n)	O(1)	Medium	Curated Top 75, dict tree, LC 211, trie, amazon,fb	AGAIN************** (7) (MUST)
211	Design Add and Search Words Data Structure	Python, Java	O(min(n, h))	O(min(n, h))	Medium	Curated Top 75, Trie, recursive, node, hashMap, do # 208 first,amazon ,fb	AGAIN*********** (8) (MUST)
226	Invert Binary Tree	Python, Java	O(n)	O(h), O(w)	Easy	Curated Top 75, good basic	OK******* (4) (but again)
297	Serialize and Deserialize Binary Tree	Python, Java	O(n)	O(h), O(w)	Hard	Curated Top 75, binary treee	again
307	Range Sum Query - Mutable	Python	ctor: O(n), update: O(logn), query: O(logn)	O(n)	Medium	LintCode, DFS, Segment Tree, BIT, check #303 Range Sum Query - Immutable	AGAIN* (2)(not start)
536	Construct Binary Tree from String	Python, Java	O(n)	O(h)	Medium	🔒, tree, recursive, trick, check # 606 Construct String from Binary Tree, amazon	AGAIN******** (5)
538	Convert BST to Greater Tree	Python, Java	O(n)	O(h)	Medium	good basic, dfs, bfs, tree, amazon	OK********* (6) (but again!)
543	Diameter of Binary Tree	Python, Java	O(n)	O(h)	Easy	dfs, tree, trick,google,amazon,fb	AGAIN********* (4)
545	Boundary of Binary Tree	Python	O(n)	O(h)	Medium	recursive, dfs, tree boundary, tree,🔒, good trick, amazon	AGAIN********* (6)
548	Split Array with Equal Sum	Python	O(n^2)	O(n)	Medium	🔒, good trick, array	AGAIN*** (2)
563	Binary Tree Tilt	Python	O(n)	O(n)	Easy		AGAIN
572	Subtree of Another Tree	Python, Java	O(m * n)	O(h)	Easy	Curated Top 75, LC 100, recursive call recursion func, good basic, dfs, bfs, amazon,fb	OK************* (10) (but again)
606	Construct String from Binary Tree	Python	O(n)	O(h)	Easy	good basic, tree, dfs, LC # 536, amazon	OK******** (7) (again!)
617	Merge Two Binary Trees	Python, Java	O(n)	O(h)	Easy	tree, dfs, bfs, good basic, amazon	OK************* (7) (but again)
623	Add One Row to Tree	Python	O(n)	O(h)	Medium	good basic, dfs, bfs	AGAIN** (2)
637	Average of Levels in Binary Tree	Python, Java	O(n)	O(h)	Easy	bfs, dfs, good basic, fb	OK*** (4)
652	Find Duplicate Subtrees	Python	O(n)	O(n)	Medium	AGAIN, good basic,dfs, Hash, amazon	AGAIN************** (6)
653	Two Sum IV - Input is a BST	Python, Java	O(n)	O(h)	Easy	Two Pointers,2 sum,bfs, dfs,amazon,fb	OK******* (4)
654	Maximum Binary Tree	Python, Java	O(n)	O(n)	Medium	LintCode, Descending Stack, good basic	AGAIN* (2)
655	Print Binary Tree	Python	O(n)	O(h)	Medium		AGAIN* (2) (not start)
662	Maximum Width of Binary Tree	Python	O(n)	O(h)	Medium	width of tree, bfs, dfs, trick, amazon	AGAIN********* (5)
663	Equal Tree Partition	Python	O(n)	O(n)	Medium	# LC 508, AGAIN, 🔒 , Hash, tree, dfs ,trick, good trick, amazon	OK******** (8)
677	Map Sum Pairs	Python	O(n)	O(t)	Medium	Trie, trick, hard	AGAIN* (2) (not start)
684	Redundant Connection	Python, Java	O(n)	O(n)	Medium	dfs, graph, Union Find, basic	AGAIN****** (3) (not start)
687	Longest Univalue Path	Python, Java	O(n)	O(h)	Medium	basic, dfs, tree, trick, google	AGAIN**** (4)
814	Binary Tree Pruning	Python	O(n)	O(h)	Medium	DFS, basic	AGAIN* (2)
863	All Nodes Distance K in Binary Tree	Python	O(n)	O(n)	Medium	LC 752, tree, DFS + BFS, trick, hard, amazon	AGAIN************ (4)
866	Smallest Subtree with all the Deepest Nodes	Python	O(n)	O(h)	Medium	DFS, trick, basic	AGAIN** (2)
889	Construct Binary Tree from Preorder and Postorder Traversal	Python, Java	O(n)	O(h)	Medium	DFS, stack, trick, tree, recursive, google	AGAIN** (2) (not start*)
897	Increasing Order Search Tree	Python	O(n)	O(h)	Easy	DFS, good basic,inorder	AGAIN** (2)
919	Complete Binary Tree Inserter	Python	ctor: O(n) insert: O(1) get_root: O(1)	O(n)	Medium	bst	AGAIN** (2)
938	Range Sum of BST	Python	O(n)	O(h)	Medium	DFS, check # 108 Convert Sorted Array to Binary Search Tree	AGAIN* (2)
951	Flip Equivalent Binary Trees	Python, Java	O(n)	O(h)	Medium	recursion, tree, DFS, google	AGAIN***s (3)
958	Check Completeness of a Binary Tree	Python	O(n)	O(w)	Medium	BFS, DFS, basic	AGAIN** (2)
965	Univalued Binary Tree	Python	O(n)	O(h)	Easy	DFS, BFS, good basic	AGAIN** (2)
971	Flip Binary Tree To Match Preorder Traversal	Python	O(n)	O(h)	Medium	DFS, trick	AGAIN** (2) (not start)
979	Distribute Coins in Binary Tree	Python	O(n)	O(h)	Medium	DFS, trick	AGAIN*** (2)
508	Most Frequent Subtree Sum	Python			Medium	tree, dfs, bfs, good basic,amazon	OK*********** (4) (but again MUST)
683	K Empty Slots	Python			Hard	BST, tree, bucket, google	not start
701	Insert into a Binary Search Tree	Python			Medium	good basic, BST, tree, BFS, DFS	OK*** (2)
1032	Stream of Characters	Java			Hard	trie, google	AGAIN (not start)
1339	Maximum Product of Splitted Binary Tree	Python			Medium	dfs, tree, amazon	AGAIN*** (1) (not start)
1372	Longest ZigZag Path in a Binary Tree	Python			Medium	dfs, bfs, post order, good trick, tree, amazon	AGAIN*** (1) (not start)
1448	Count Good Nodes in Binary Tree	Python, Java			Medium	max in path, dfs, bfs, good trick, tree, needcode, google, amz, m$	AGAIN********* (1) (MUST)

Heap

#	Title	Solution	Time	Space	Difficulty	Note	Status
264	Ugly Number II	Python	O(n)	O(1)	Medium	good trick, LC 263, LC 313, Heap, LintCode , DP, M$	AGAIN*********** (4) (MUST)
313	Super Ugly Number	Python	O(n * k)	O(n + k)	Medium	BST, Heap	AGAIN (not start*)
373	Find K Pairs with Smallest Sums	Python	O(k * log(min(n, m, k)))	O(min(n, m, k))	Medium		AGAIN (not start)
378	Kth Smallest Element in a Sorted Matrix	Python	O(k * log(min(n, m, k)))	O(min(n, m, k))	Medium	LintCode	AGAIN (not start)
846	Hand of Straights	Python, Java	O(nlogn)	O(n)	Medium	LC 1296, good basic, Treemap, hashmap, google	OK*** (2)
855	Exam Room	Python, Java	seat: O(logn) leave: O(logn)	O(n)	Medium	treeSet, BST, Hash, trick, google	AGAIN* (3) (not start)
295	Find Median from Data Stream	Python, Java			Hard	Curated Top 75, priority queue, trick ,heap, stream, amazon	AGAIN****** (6)
703	Kth Largest Element in a Stream	Python, Java			Easy	heap, priority queue, amazon	AGAIN***** (4)
1046	Last Stone Weight	Python, Java			Easy	Heap, Priority Queue	OK
1130	Minimum Cost Tree From Leaf Values	Python			Medium	LC 1167,heap	not start
1167	Minimum Cost to Connect Sticks	Python			Medium	LC 1130, good basic,heap,amazon	AGAIN***** (3)
1353	Maximum Number of Events That Can Be Attended	Python, Java			Medium	LC 252, 253, good trick, meeting room, heap, PRIORITY QUEUE, fb, twitter, amazon	AGAIN********* (3) MUST
1481	Least Number of Unique Integers after K Removals	Python			Medium	good trick, greedy, Counter, dict, heap,amazon	AGAIN******** (3)

Bit Manipulation

#	Title	Solution	Time	Space	Difficulty	Note	Status
136	Single Number	Python	O(n)	O(1)	Easy	airbnb,amazon,fb	OK
137	Single Number II	Python	O(n)	O(1)	Medium		OK
190	Reverse Bits	Python, Java	O(1)	O(1)	Easy	Curated Top 75, bit, apple	AGAIN*** (2)
191	Number of 1 Bits	Python, Java	O(1)	O(1)	Easy	Curated Top 75, apple	OK*
201	Bitwise AND of Numbers Range	Python	O(1)	O(1)	Medium		AGAIN (not start)
231	Power of Two	Python	O(1)	O(1)	Easy	good basic, Bit Manipulation ,amazon	OK*** (3)
260	Single Number III	Python	O(n)	O(1)	Medium	xor, ^=, ^	AGAIN*
268	Missing Number	Python, Java	O(n)	O(1)	Medium	Curated Top 75, xor,amazon,fb	OK* (2)
318	Maximum Product of Word Lengths	Python	O(n) ~ O(n^2)	O(n)	Medium	Bit Manipulation, Counting Sort, Pruning	AGAIN (not start)
342	Power of Four	Python	O(1)	O(1)	Easy	power of num	OK*
371	Sum of Two Integers	Python, Java	O(1)	O(1)	Medium	Curated Top 75, good basic, fb	AGAIN********* (5) (bit op not start)
389	Find the Difference	Python	O(n)	O(1)	Easy	amazon	OK (2)
393	UTF-8 Validation	Python	O(n)	O(1)	Medium	utf-8 encoding, google, fb	AGAIN**** (not start*) (5)
401	Binary Watch	Python	O(1)	O(1)	Easy		OK*
421	Maximum XOR of Two Numbers in an Array	Python	O(n)	O(n)	Medium		AGAIN (not start)
461	Hamming Distance	Python	O(1)	O(1)	Easy	fb	OK* (2)
462	Minimum Moves to Equal Array Elements II	Python	O(n) on average	O(1)	Medium		AGAIN*
477	Total Hamming Distance	Python	O(n)	O(1)	Medium	bit manipulation, fb	AGAIN****** (not start*) (4)
645	Set Mismatch	Python	O(n)	O(1)	Easy	amazon	OK* (3)
693	Binary Number with Alternating Bits	Python	O(1)	O(1)	Easy	trick	AGAIN*
762	Prime Number of Set Bits in Binary Representation	Python	O(1)	O(1)	Easy	basic,Bit_Manipulation, prime number, amazon	OK* (4)
868	Binary Gap	Python	O(1)	O(1)	Easy	trick, linear scan	AGAIN*
898	Bitwise ORs of Subarrays	Python	O(n)	O(1)	Medium		AGAIN (not start)
1573	Number of Ways to Split a String	Python	O(n)	O(1)	Medium	amazon, binary	AGAIN** (not start)
1915	Number of Wonderful Substrings	Python	O(n)	O(1)	Medium	bit mask, bit, prefix sum, dp, amazon	AGAIN*** (2) (not start)

String

#	Title	Solution	Time	Space	Difficulty	Status, Note	Status
005	Longest Palindromic Substring	Python, Java	O(n)	O(n)	Medium	Curated Top 75, LC 647, good trick, 2 pointer, recursive, Manacher's Algorithm, LC 100 like, DP, UBER, amazon, fb	AGAIN************** (8)(MUST)
006	ZigZag Conversion	Python	O(n)	O(1)	Medium		AGAIN (not start)
008	String to Integer (atoi)	Python	O(n)	O(1)	Medium	string op, regular expression,UBER, amazon, fb	AGAIN***** (5)
014	Longest Common Prefix	Python	O(n * k)	O(1)	Easy	good basic	OK*
028	Implement strStr()	Python, Java	O(n + k)	O(k)	Easy	KMP Algorithm, fb	OK (3)
038	Count and Say	Python, Java	O(n * 2^n)	O(2^n)	Medium	recursion, iteration, basic, fb	AGAIN*********** (5)
043	Multiply Strings	Python, Java	O(m * n)	O(m + n)	Medium	twitter, fb	AGAIN*** (3)
058	Length of Last Word	Python	O(n)	O(1)	Easy		OK
067	Add Binary	Python, Java	O(n)	O(1)	Easy	bit op, good basic, fb, amazon	OK***** (7) (but again)
076	Minimum Window Substring	Python, Java			Hard	Curated Top 75	AGAIN******* (2) (not start)
125	Valid Palindrome	Python, Java	O(n)	O(1)	Easy	Curated Top 75, LC 680, amazon,fb	OK
151	Reverse Words in a String	Python	O(n)	O(1)	Medium	amazon,fb	OK (2)
161	One Edit Distance	Python	O(m + n)	O(1)	Medium	🔒, trick, recursion, string, , fb	AGAIN ******** (6)(AGAIN)
165	Compare Version Numbers	Python	O(n)	O(1)	Medium	good basic, string,amazon, apple	OK** (5)
186	Reverse Words in a String II	Python	O(n)	O(1)	Medium	LC 151, LC 557, 🔒, M$,UBER, amazon	AGAIN* (3)
242	Valid Anagram	Python, Java	O(n)	O(1)	Easy	Curated Top 75, LintCode,amazon,fb	OK
271	Encode and Decode Strings	Python, Java	O(n)	O(1)	Medium	Curated Top 75, decode-encode,string op, check # 394 Decode String ,🔒, google	AGAIN*** (3)
306	Addictive Number	Python	O(n^3)	O(n)	Medium		AGAIN (not start*)
340	Longest Substring with At Most K Distinct Characters	Python	O(n^3)	O(n)	Hard	string, sliding window, google	not start
383	Ransom Note	Python	O(n)	O(1)	Easy	EPI, apple	OK*
405	Convert a Number to Hexadecimal	Python	O(n)	O(1)	Easy	basics decimal -> Hexadecimal	AGAIN
408	Valid Word Abbreviation	Python	O(n)	O(1)	Easy	🔒	AGAIN (not start)
415	Add Strings	Python	O(n)	O(1)	Easy	good basic, add xxx to sum, Airbnb, google, fb	OK****** (4)
434	Number of Segments in a String	Python	O(n)	O(1)	Easy		OK
443	String Compression	Python	O(n)	O(1)	Easy	shoptify, basics	AGAIN**
459	Repeated Substring Pattern	Python	O(n)	O(n)	Easy	good basic, string, KMP Algorithm, amazon	OK*** (6)
468	Validate IP Address	Python	O(1)	O(1)	Medium	ip basic,string,twitter, fb, amazon	AGAIN****** (5)
482	License Key Formatting	Python, Java	O(n)	O(1)	Easy	string, basic, google	OK** (4) (again)
520	Detect Capital	Python	O(l)	O(1)	Easy		OK
521	Longest Uncommon Subsequence I	Python	O(min(a, b))	O(1)	Easy		OK*
522	Longest Uncommon Subsequence II	Python	O(l * n^2)	O(1)	Medium	Sort	AGAIN (not start)
524	Longest Word in Dictionary through Deleting	Python, Java	O((d * l) * logd)	O(1)	Medium Sort, dict, brute force, google	AGAIN (not start)
539	Minimum Time Difference	Python	O(nlogn)	O(n)	Medium	basic, map & zip trick	AGAIN**
541	Reverse String II	Python	O(n)	O(1)	Easy		AGAIN*
551	Student Attendance Record I	Python	O(n)	O(1)	Easy		OK
556	Next Greater Element III	Python	O(1)	O(1)	Medium		AGAIN (not start)
557	Reverse Words in a String III	Python	O(n)	O(1)	Easy		OK
616	Add Bold Tag in String	Python	O(n * d * l)	O(n)	Medium	🔒, google	AGAIN (not start)
647	Palindromic Substrings	Python, Java	O(n)	O(n)	Medium	Curated Top 75, LC 005, greedy, DP, expand from center, 2 pointers, good basic,Manacher's Algorithm, Linkedin,amazon,fb	OK********* (8) (MUST)
648	Replace Words	Python	O(n)	O(t)	Medium	Trie, good basic	OK*
657	Judge Route Circle	Python	O(n)	O(1)	Easy		OK
678	Valid Parenthesis String	Python	O(n)	O(1)	Medium		AGAIN**
680	Valid Palindrome II	Python	O(n)	O(1)	Easy	LC 125, two pointers, good basic, Palindrome, string, fb	AGAIN******** (3)
681	Next Closest Time	Python	O(1)	O(1)	Medium	subset of set, <=,strptime, google,fb	OK*** (5)
686	Repeated String Match	Python, Java	O(n + m)	O(1)	Medium	brute force, math, string, Rabin-Karp Algorithm, google	AGAIN**** (3)
696	Count Binary Substrings	Python	O(n)	O(1)	Easy	good trick, linear scan, group sub-string, amazon	AGAIN******* (3) (MUST)
720	Longest Word in Dictionary	Python	O(n)	O(t)	Easy	Trie	AGAIN (not start)
722	Remove Comments	Python	O(n)	O(k)	Medium		AGAIN (not start)
734	Sentence Similarity	Python , Java	O(n + p)	O(p)	Easy	good basic, graph, hash table,collections.defaultdict(set) ,google	OK***** (5)
751	IP to CIDR	Python	O(n)	O(1)	Medium	ip	AGAIN (not start*)
758	Bold Words in String	Python	O(n * l)	O(t)	Easy	🔒, variant of Add Bold Tag in String	AGAIN (not start)
791	Custom Sort String	Python	O(n)	O(1)	Medium	good basic, sort, counter, amazon, fb	AGAIN************ (7) (MUST)
796	Rotate String	Python	O(n)	O(1)	Easy	good basic, KMP Algorithm, Rabin-Karp Algorithm,amazon	OK**** (2)(but again)
804	Unique Morse Code Words	Python	O(n)	O(n)	Easy		OK*
806	Number of Lines To Write String	Python	O(n)	O(1)	Easy		OK*
809	Expressive Words	Python, Java	O(n + s)	O(l + s)	Medium	google, string	AGAIN (1)
816	Ambiguous Coordinates	Python	O(n^4)	O(n)	Medium		AGAIN (not start*)
819	Most Common Word	Python	O(m + n)	O(m + n)	Easy	regular expression, amazon	OK** (2)
820	Short Encoding of Words	Python	O(n)	O(t)	Medium	Trie	AGAIN (not start)
824	Goat Latin	Python	O(n + w^2)	O(l)	Easy	string basic, fb	OK
828	Count Unique Characters of All Substrings of a Given String	Python	O(n + w^2)	O(l)	Hard	LC 1248, dp, string, amazon	AGAIN*** (2)
831	Masking Personal Information	Python	O(1)	O(1)	Medium	regular expression	OK*
833	Find And Replace in String	Python	O(n + m)	O(n)	Medium	Union Find	AGAIN (not start)
848	Shifting Letters	Python	O(n)	O(1)	Medium	remainder, basic	AGAIN**
859	Buddy Strings	Python	O(n)	O(1)	Easy		AGAIN*
880	Decoded String at Index	Python	O(n)	O(1)	Medium	trick	AGAIN (not start**)
884	Uncommon Words from Two Sentences	Python	O(m + n)	O(m + n)	Easy	collections Counter, trick	OK*
890	Find and Replace Pattern	Python	O(n * l)	O(1)	Medium		AGAIN (not start)
893	Groups of Special-Equivalent Strings	Python	O(n * l)	O(n)	Easy		AGAIN*
916	Word Subsets	Python	O(m + n)	O(1)	Medium		AGAIN (not start)
917	Reverse Only Letters	Python	O(n)	O(1)	Easy	basic,stack,pointer	OK*
925	Long Pressed Name	Python	O(n)	O(1)	Easy	two pointer, basic, trick	AGAIN*
929	Unique Email Addresses	Python	O(n * l)	O(n * l)	Easy		OK
939	Minimum Area Rectangle	Python, Java	O(n^1.5) on average	O(n)	Medium	hashmap, brute force, google	AGAIN !!! (2)
942	DI String Match	Python	O(n)	O(1)	Easy		AGAIN (not start)
944	Delete Columns to Make Sorted	Python	O(n * l)	O(1)	Medium		AGAIN (not start)
953	Verifying an Alien Dictionary	Python	O(n * l)	O(1)	Easy		AGAIN (not start)
955	Delete Columns to Make Sorted II	Python	O(n * l)	O(n)	Medium		AGAIN (not start)
1119	Remove Vowels from a String	Python	O(n * l)	O(n)	Easy	amazon	OK

Queue

#	Title	Solution	Time	Space	Difficulty	Tag	Note
281	Zigzag Iterator	Python	O(n)	O(k)	Medium	good basic, queue, 🔒, google,fb	AGAIN******* (5)
346	Moving Average from Data Stream	Python, Java	O(1)	O(w)	Easy	🔒, queue, google, fb	OK** (6)
933	Number of Recent Calls	Python	O(1) on average	O(w)	Easy		AGAIN**
622	Design Circular Queue	Python			Medium	good basic, array, linked list, design, circular queue,amazon,fb, apple, airbnb, m$	AGAIN************ (3) (MUST)
239	Sliding Window Maximum	Python			Hard	sliding window, heap, deque, queue,amazon, google	AGAIN********* (4)

Math

#	Title	Solution	Time	Space	Difficulty	Note	Status
007	Reverse Integer	Python	O(1)	O(1)	Medium	0x7FFFFFFF, amazon, apple	OK* (3)
009	Palindrome Number	Python	O(1)	O(1)	Easy	amazon	OK
012	Integer to Roman	Python	O(n)	O(1)	Medium	math basic,twitter, fb, amazon, math	AGAIN****** (4)
013	Roman to Integer	Python	O(n)	O(1)	Easy	UBER,amazon, fb	AGAIN***** (4)
029	Divide Two Integers	Python	O(1)	O(1)	Medium	binary search,trick,fb	AGAIN***** (3)
050	Pow(x, n)	Python, Java	O(1)	O(1)	Medium	good trick,amazon, fb, recursion	AGAIN****** (4)
060	Permutation Sequence	Python	O(n^2)	O(n)	Medium	Cantor Ordering,trick ,amazon	AGAIN*** (2)
089	Gray Code	Python	O(2^n)	O(1)	Medium	bit op, math, amazon	AGAIN ********** (6)
166	Fraction to Recurring Decimal	Python	O(logn)	O(1)	Medium		AGAIN (not start)
168	Excel Sheet Column Title	Python	O(logn)	O(1)	Easy	trick, residual, math, AGAIN,fb	AGAIN****** (5)
171	Excel Sheet Column Number	Python	O(n)	O(1)	Easy		AGAIN (not start)
172	Factorial Trailing Zeroes	Python	O(1)	O(1)	Easy	basic, iteration, recursion	AGAIN* (2)
223	Rectangle Area	Python	O(1)	O(1)	Easy		AGAIN
258	Add Digits	Python	O(1)	O(1)	Easy		AGAIN
263	Ugly Number	Python	O(1)	O(1)	Easy	math	OK* (2)
273	Integer to English Words	Python	O(1)	O(1)	Hard	amazon, passed	not start
292	Nim Game	Python	O(1)	O(1)	Easy	LintCode	OK*
319	Bulb Switcher	Python	O(1)	O(1)	Medium		AGAIN
326	Power of Three	Python	O(1)	O(1)	Easy		OK
338	Counting Bits	Python, Java	O(n)	O(n)	Medium	Curated Top 75	OK
343	Integer Break	Python	O(logn)	O(1)	Medium	trick, DP	AGAIN* (2)
365	Water and Jug Problem	Python	O(logn)	O(1)	Medium	Bézout's identity	AGAIN (not start)
372	Super Pow	Python	O(n)	O(1)	Medium		AGAIN
382	Linked List Random Node	Python	O(n)	O(1)	Medium	Reservoir Sampling	AGAIN (not start)
386	Lexicographical Numbers	Python	O(n)	O(1)	Medium		AGAIN (not start)
390	Elimination Game	Python	O(logn)	O(1)	Medium		AGAIN (not start)
398	Random Pick Index	Python	O(n)	O(1)	Medium	Reservoir Sampling , fb	OK* (4)
400	Nth Digit	Python	O(logn)	O(1)	Easy		AGAIN
413	Arithmetic Slices	Python	O(n)	O(1)	Medium	sliding window, amazon	OK*** (1) (but again)
423	Reconstruct Original Digits from English	Python	O(n)	O(1)	Medium	GCJ2016 - Round 1B	AGAIN (not start)
441	Arranging Coins	Python	O(nlogn)	O(1)	Easy	Binary Search	OK
453	Minimum Moves to Equal Array Elements	Python	O(n)	O(1)	Easy		AGAIN
458	Poor Pigs	Python	O(n)	O(1)	Easy		AGAIN
469	Convex Polygon	Python	O(n)	O(1)	Medium	🔒	AGAIN (not start)
470	Implement Rand10() Using Rand7()	Python	O(1)	O(1)	Medium		AGAIN (not start)
478	Generate Random Point in a Circle	Python	O(1)	O(1)	Medium		AGAIN (not start)
497	Random Point in Non-overlapping Rectangles	Python	ctor: O(n) pick: O(logn)	O(n)	Medium		AGAIN (not start)
504	Base 7	Python	binary, math, good basic	O(n)	Easy		AGAIN******** (1)
519	Random Flip Matrix	Python	ctor: O(1) pick: O(1) reset: O(n)	O(n)	Medium		AGAIN (not start)
528	Random Pick with Weight	Python, Java	O(logn)	O(n)	Medium	good basic, binary search, prefix sum, google	AGAIN**** (1)
537	Complex Number Multiplication	Python	O(1)	O(1)	Medium	math, amazon	OK (3)
553	Optimal Division	Python	O(n)	O(1)	Medium	string, math, amazon	OK* (4)
573	Squirrel Simulation	Python	O(n)	O(1)	Medium	🔒	AGAIN (not start)
592	Fraction Addition and Subtraction	Python	O(nlogx)	O(n)	Medium		AGAIN
593	Valid Square	Python	O(1)	O(1)	Medium		AGAIN
598	Range Addition II	Python	O(p)	O(1)	Easy		AGAIN
625	Minimum Factorization	Python	O(loga)	O(1)	Medium	🔒	OK*
628	Maximum Product of Three Numbers	Python	O(n)	O(1)	Easy	amazon	OK (2)
633	Sum of Square Numbers	Python	O(sqrt(c) * logc)	O(1)	Easy		OK*
634	Find the Derangement of An Array	Python	O(n)	O(1)	Medium	🔒	AGAIN (not start)
640	Solve the Equation	Python	O(n)	O(n)	Medium	math, regular expression, amazon	AGAIN*** (3)
651	4 Keys Keyboard	Python	O(1)	O(1)	Medium	🔒, Math, DP	AGAIN (not start)
672	Bulb Switcher II	Python	O(1)	O(1)	Medium		AGAIN (not start)
728	Self Dividing Numbers	Python	O(n)	O(1)	Medium		AGAIN
754	Reach a Number	Python	O(logn)	O(1)	Medium		OK*
775	Global and Local Inversions	Python	O(n)	O(1)	Medium	trick, math, string, amazon	AGAIN** (not start) (3)
779	K-th Symbol in Grammar	Python	O(1)	O(1)	Medium	math, brute force, binary, recursion, FB, amazon	AGAIN*** (1)
781	Rabbits in Forest	Python	O(n)	O(n)	Medium		AGAIN (not start)
789	Escape The Ghosts	Python	O(n)	O(1)	Medium		AGAIN
800	Similar RGB Color	Python	O(1)	O(1)	Easy	🔒	AGAIN (not start)
812	Largest Triangle Area	Python	O(n^3)	O(1)	Easy		AGAIN (not start)
829	Consecutive Numbers Sum	Python	O(sqrt(n))	O(1)	Medium		AGAIN
836	Rectangle Overlap	Python	O(1)	O(1)	Easy	similar as 223. Rectangle Area, amazon	AGAIN (2)
858	Mirror Reflection	Python	O(1)	O(1)	Medium		AGAIN (not start)
867	Prime Palindrome	Python	O(n^(1/2) * (logn + n^(1/2)))	O(logn)	Medium		AGAIN***
883	Projection Area of 3D Shapes	Python	O(n^2)	O(1)	Easy		AGAIN
891	Sum of Subsequence Widths	Python	O(n^2)	O(1)	Hard	amazon	AGAIN (not start)
907	Sum of Subarray Minimums	Python	O(n)	O(n)	Medium	trick, LC 084, LC 2104, Ascending Stack, amazon	AGAIN**** (1)
908	Smallest Range I	Python	O(n)	O(1)	Easy		OK* (2)
910	Smallest Range II	Python	O(nlogn)	O(1)	Medium		AGAIN (not start)
914	X of a Kind in a Deck of Cards	Python	O(n * (logn)^2)	O(n)	Easy		AGAIN (not start)
963	Minimum Area Rectangle II	Python	O(n^2) ~ O(n^3)	O(n^2)	Medium		AGAIN (not start)
970	Powerful Integers	Python	O((logn)^2)	O(r)	Easy	basic	AGAIN* (2)
517	Super Washing Machines	Python			Hard	amazon	AGAIN (not start)
1342	Number of Steps to Reduce a Number to Zero	Python			easy	gra*	ok
1492	The kth Factor of n	Python			Medium	amazon	AGAIN (not start)
1979	Find Greatest Common Divisor of Array	Python			Easy	GCD	not start
2013	Detect Squares	Java			Medium	math	not start
3190	Find Minimum Operations to Make All Elements Divisible by Three	Java			Easy	LC weekly, math	OK
3194	Minimum Average of Smallest and Largest Elements	Java			Easy	LC weekly, math	OK

Sort

#	Title	Solution	Time	Space	Difficulty	Note	Status
075	Sort Colors	Python	O(n)	O(1)	Medium	sorting, Tri Partition,two pointers, M$, fb	OK** (4)
088	Merge Sorted Array	Python, Java	O(n)	O(1)	Easy	good basic, merge, sort,two pointers, M$, amazon,fb	AGAIN************* (8) (MUST)
128	Longest Consecutive Sequence	Python, Java			Medium	Curated Top 75, 2 pointers, sort, sliding window, amazon	AGAIN*********** (4) (MUST)
147	Insertion Sort List	Python	O(n^2)	O(1)	Medium	trick	AGAIN* (2)
148	Sort List	Python	O(nlogn)	O(logn)	Medium	basic, check # 21 Merge Two Sorted Lists	AGAIN** (2)
179	Largest Number	Python	O(nlogn)	O(1)	Medium	good basic, sort with lambda,amazon	AGAIN************ (3)
252	Meeting Rooms	Python, Java	O(nlogn)	O(n)	Easy	Curated Top 75, sort, 🔒,UBER,amazon, fb	OK*** (4) (but again)
253	Meeting Rooms II	Python, Java	O(nlogn)	O(n)	Medium	Curated Top 75, 🔒, sort, priority queue (min-heap),scanning line, trick,booking.com,good,UBER, amazon,google,fb	AGAIN************ (5) (MUST)
274	H-Index	Python	O(n)	O(n)	Medium	Counting Sort,good trick, fb	AGAIN***** (3)
280	Wiggle Sort	Python	O(n)	O(1)	Medium	🔒,google,fb	OK*** (3)
324	Wiggle Sort II	Python	O(n) on average	O(1)	Medium	variant of Sort Colors , Tri Partition, google	AGAIN** (2)
347	Top K Frequent Elements	Python, Java	O(n)	O(n)	Medium	Curated Top 75 , HashMap, Heap, Sort,Yelp,amazon,fb, GOOD basic	AGAIN******** (4)
406	Queue Reconstruction by Height	Python, Java	O(n * sqrt(n))	O(n)	Medium	basic,trick, sort by key, insert, google	AGAIN********* (5)
451	Sort Characters By Frequency	Python	O(n)	O(n)	Medium	collections.Counter(s).most_common,sorted(count_dict.items() with lambda,good basic, amazon	OK* (4)
692	Top K Frequent Words	Python	O(n + klogk) on average	O(n)	Medium	good basic,Quick Select, Heap, Bucket Sort, heapq, yelp, UBER,amazon,fb	OK***** (5)
937	Reorder Data in Log Files	Python	O(nlogn * l)	O(l)	Medium	good basic, sort, string, amazon	OK**** (3)(again!!!)
969	Pancake Sorting	Python	O(n^2)	O(l)	Medium	fb, sorting	AGAIN********** (4)
973	K Closest Points to Origin	Python, Java	O(n) on average	O(1)	Medium	sort with lambda, Quick Select, Heap, amazon	OK* (3)
976	Largest Perimeter Triangle	Python, Java	O(nlogn)	O(1)	Easy		OK* (2)
912	Sort an Array	Python	O(nlogn)	O(1)	Medium	sort, merge sort, quick sort, apple, amazon, m$	AGAIN** (1)
1057	Campus Bike	Java	O(nlogn)	O(1)	Medium	sorting, google	AGAIN
1066	Campus Bike II	Java	O(nlogn)	O(1)	Medium	sorting, google	AGAIN
1152	Analyze User Website Visit Pattern	Python	O(nlogn)	O(1)	Medium	itertools.combinations, sort, brute force, set, amazon	AGAIN***** (2) (not start)
2021	Brightest Position on Street	Python	O(nlogn)	O(1)	Medium	LC 253 meeting room II, priority queue (min-heap), scanning line, trick, amazon	AGAIN*********** (3) (MUST)

Two Pointers

#	Title	Solution	Time	Space	Difficulty	Note	Status
019	Remove Nth Node From End of List	Python, Java	O(n)	O(1)	Medium	Curated Top 75, good basic, MUST, linked list,two pointers,fb	AGAIN********** (4) (MUST)
042	Trapping Rain Water	Python, Java			Hard	scan, dp, trick, two pointer, amazon, apple	AGAIN*** (2) (not start)
086	Partition List	Python	O(n)	O(1)	Medium		OK*
141	Linked List Cycle	Python, Java	O(n)	O(1)	Easy	Curated Top 75, basic,amazon, fb, linked list, 2 pointers, check #142	OK** (5)
142	Linked List Cycle II	Python	O(n)	O(1)	Medium	linked list, 2 pointers, check #141	AGAIN****** (4) (again!)
143	Reorder List	Python, Java	O(n)	O(1)	Medium	Curated Top 75, LC 021, 206, 876, good trick,inverse linkedlist, merge linkedlist ,fb, amazon	AGAIN********** (5) (MUST)
167	Two Sum II - Input array is sorted	Python, Java	O(n)	O(1)	Medium	two pointer, dict, binary search,good basic, amazon	OK* (6)
259	3Sum Smaller	Python	O(n^2)	O(1)	Medium	🔒, LintCode, good trick, google, fb, check #015 3 Sum, check #001 two sum	AGAIN******** (4)
283	Move Zeroes	Python, Java	O(n)	O(1)	Easy	basic, trick, ,two pointers,Shopee,fb	AGAIN** (5)
344	Reverse String	Python	O(n)	O(1)	Easy		OK
345	Reverse Vowels of a String	Python, Java	O(n)	O(1)	Easy	google	OK** (1)
349	Intersection of Two Arrays	Python	O(m + n)	O(min(m, n))	Easy	EPI, Hash, Binary Search, fb	OK
350	Intersection of Two Arrays II	Python	O(m + n)	O(1)	Easy	EPI, Hash, Binary Search, good basic, fb	OK** (3)
360	Sort Transformed Array	Python	O(n)	O(1)	Medium	trick,🔒,google ,fb	AGAIN*** (3)
457	Circular Array Loop	Python	O(n)	O(1)	Medium		AGAIN (not start)
567	Permutation in String	Python, Java	O(n)	O(1)	Medium	Collections.counter,sliding window, sorting, hashmap,substring, good basic,M$, fb	AGAIN********* (8)
611	Valid Triangle Number	Python	O(n^2)	O(1)	Medium		AGAIN (not start)
777	Swap Adjacent in LR String	Python	O(n)	O(1)	Medium		AGAIN (not start)
826	Most Profit Assigning Work	Python	O(mlogm + nlogn)	O(n)	Medium	trick, good basic, zip+sorted	OK*
844	Backspace String Compare	Python, Java	O(m + n)	O(1)	Easy	good basic,stack,google, fb	OK* (4)
876	Middle of the Linked List	Python, Java	O(n)	O(1)	Easy	basic, amazon, apple, google, 2 pointers, linkedlist	OK* (2)
904	Fruit Into Baskets	Python	O(n)	O(1)	Medium		AGAIN (not start)
930	Binary Subarrays With Sum	Python	O(n)	O(1)	Medium	trick	AGAIN* (2) (not start)
977	Squares of a Sorted Array	Python	O(n)	O(1)	Easy		OK
1055	Shortest Way to Form String	Python, Java	O(n)	O(h)	Medium	2 pointer, google	again
1151	Minimum Swaps to Group All 1's Together	Python	O(n)	O(1)	Medium	good basic, 2 pointers, Deque (Double-ended Queue), amazon	AGAIN***** (not start )
1963	Minimum Number of Swaps to Make the String Balanced	Python	O(n)	O(h)	Medium	good basic, greedy, stack, 2 pointers, amazon, fb, m$	AGAIN******** (2)
2104	Sum of Subarray Ranges	Python	O(n)	O(1)	Medium	LC 907, monotonic stack, brute force, 2 pointers, dp, amazon	AGAIN******* (3)
3191	Minimum Operations to Make Binary Array Elements Equal to One I	Java	O(n)	O(h)	Medium	2 pointers	Again* (2)
3192	Minimum Operations to Make Binary Array Elements Equal to One II	Java	O(n)	O(h)	Medium	2 pointers	Again* (2)

Recursion

#	Title	Solution	Time	Space	Difficulty	Note	Status
10	Regular Expression Matching	Python	O(n)	O(h)	Hard	recursive, dp, amazon	AGAIN (not start)
095	Unique Binary Search Trees II	Python	O(4^n / n^(3/2)	O(4^n / n^(3/2)	Medium	recursion, BST, amazon, google, apple	AGAIN!!! (1)(good trick)
098	Validate Binary Search Tree	Python, Java	O(n)	O(1)	Medium	Curated Top 75, recursion, bfs, dfs,BST,M$,amazon, fb	AGAIN****************** (11)
100	Same Tree	Python, Java	O(n)	O(h)	Easy	Curated Top 75, recursion, dfs, bfs, good basic,amazon	OK****** (5) (but again)
104	Maximum Depth of Binary Tree	Python, Java	O(n)	O(h)	Easy	Curated Top 75, DFS, BFS, good basic,amazon, LC 111	OK******** (4) (but again)
105	Construct Binary Tree from Preorder and Inorder Traversal	Python, Java	O(n)	O(n)	Medium	Curated Top 75, trick, check # 106,BST,M$, fb	AGAIN********* (9)
106	Construct Binary Tree from Inorder and Postorder Traversal	Python, Java	O(n)	O(n)	Medium	trick,check # 105,BST,M$, fb	AGAIN******* (6)
108	Convert Sorted Array to Binary Search Tree	Python	O(n)	O(logn)	Medium	good concept	AGAIN*
109	Convert Sorted List to Binary Search Tree	Python	O(n)	O(logn)	Medium	list,BST,good concept, fb	AGAIN*** (3)
110	Balanced Binary Tree	Python, Java	O(n)	O(h)	Easy	good trick,dfs,amazon ,fb, google, top-down, bottom-up recursion	AGAIN************ (6)(MUST)
111	Minimum Depth of Binary Tree	Python, Java	O(n)	O(h)	Easy	good basic,dfs,BST,amazon,fb, LC 104	AGAIN******** (5)
114	Flatten Binary Tree to Linked List	Python	O(n)	O(h)	Medium	BST, dfs, M$, fb	AGAIN**** (4)
116	Populating Next Right Pointers in Each Node	Python	O(n)	O(1)	Medium	bfs, dfs, tree, recursion ,AGAIN,fb, amazon	AGAIN****** (5)
117	Populating Next Right Pointers in Each Node II	Python	O(n)	O(h)	Medium	Populating Next Right Pointers in Each Node I, bfs, linked list, amazon, fb	AGAIN****** (3)
129	Sum Root to Leaf Numbers	Python	O(n)	O(h)	Medium	trick,BST, dfs, fb	AGAIN*** (4)
156	Binary Tree Upside Down	Python	O(n)	O(1)	Medium	🔒	AGAIN (not start)
241	Different Ways to Add Parentheses	Python	O(n * 4^n / n^(3/2))	O(n * 4^n / n^(3/2))	Medium		AGAIN (not start)
298	Binary Tree Longest Consecutive Sequence	Python, Java	O(n)	O(h)	Medium	good basic, bfs, dfs, tree, 🔒, google	OK***** (3)
333	Largest BST Subtree	Python	O(n)	O(h)	Medium	🔒	AGAIN (not start)
337	House Robber III	Python	O(n) O(h)	Medium	amazon, google	AGAIN (not start)
395	Longest Substring with At Least K Repeating Characters	Python	O(n)	O(1)	Medium	AGAIN,recursion, good trick,fb	AGAIN************ (4)
404	Sum of Left Leaves	Python, Java	O(n)	O(h)	Easy, tree, bfs, dfs, amazon	AGAIN**** (4)
437	Path Sum III	Python	O(n)	O(h)	Medium	good trick,UBER, AMAZON, fb	AGAIN*** (3)
544	Output Contest Matches	Python	O(n)	O(n)	Medium		AGAIN
549	Binary Tree Longest Consecutive Sequence II	Python	O(n)	O(h)	Medium	🔒	AGAIN (not start)
669	Trim a Binary Search Tree	Python	O(n)	O(h)	Medium	good basic, BST, dfs, recursion,amazon	AGAIN******* (4)
671	Second Minimum Node In a Binary Tree	Python	O(n)	O(h)	Easy	dfs,tree	OK* (2)
753	Cracking the Safes	Java	O(n)	O(h)	Hard	dfs,recursion,google	AGAIN (not start)
1145	Binary Tree Coloring Game	Java	O(n)	O(h)	Medium	tree,recursion,google	AGAIN (not start)

Binary Search

#	Title	Solution	Time	Space	Difficulty	Tag, Note	Status
033	Search in Rotated Sorted Array	Python, Java			Medium	Curated Top 75, binary search,good basic ,check # 81 Search in Rotated Sorted Array II, binary search, rotation array, UBER,amazon,fb	AGAIN************ (6) (MUST)
034	Find First and Last Position of Element in Sorted Array	Python	O(n)	O(1)	Medium	binary search, LC top 100 like, amazon, fb, google, apple, uber	AGAIN************** (4) (MUST)
034	Search for a Range	Python	O(logn)	O(1)	Medium		AGAIN*
035	Search Insert Position	Python	O(logn)	O(1)	Medium		AGAIN
069	Sqrt(x)	Python	O(logn)	O(1)	Medium	math, binary search,amazon,fb	OK* (4)
074	Search a 2D Matrix	Python, Java	O(logm + logn)	O(1)	Medium	binary search in 2D array, flatten matrix,amazon	OK**** (BUT AGAIN)(3)
081	Search in Rotated Sorted Array II	Python	O(logn)	O(1)	Medium	binary search,check # 33 Search in Rotated Sorted Array first,array, fb, amazon, linkedin	OK***** (6)
153	Find Minimum in Rotated Sorted Array	Python, Java	O(logn)	O(1)	Medium	Curated Top 75, binary search, ascending array, good basic,amazon	AGAIN********** (5)(MUST)
162	Find Peak Element	Python, Java	O(logn)	O(1)	Medium	good trick, recursive, iterative binary search,M$, google, fb	AGAIN********** (6)
222	Count Complete Tree Nodes	Python, Java	O((logn)^2)	O(1)	Easy	google, tree	OK (again)
275	H-Index II	Python	O(logn)	O(1)	Medium	similar as # 274 H-Index, Binary Search, fb	AGAIN**** (3)
278	First Bad Version	Python, Java	O(logn)	O(1)	Easy	good basic,LintCode, binary search, fb	OK*** (5) (MUST)
300	Longest Increasing Subsequence	Python, Java	O(nlogn)	O(n)	Medium	Curated Top 75, array, binary search,DP good basic, LintCode, DP,amazon,fb	AGAIN********** (10)
315	Count of Smaller Numbers After Self	Python, Java	O(logn)	O(1)	Hard	binary search, BST, BIT, google	again*
367	Valid Perfect Square	Python, Java	O(logn)	O(1)	Easy	good basic, similar as # 69 Sqrt(x)	OK* (3)
374	Guess Number Higher or Lower	Python	O(logn)	O(1)	Easy		OK*
410	Split Array Largest Sum	Python, Java	O(logn)	O(1)	Hard	google	AGAIN (not start)
436	Find Right Interval	Python	O(nlogn)	O(n)	Medium		AGAIN (not start)
475	Heaters	Python	O((m + n) * logn)	O(1)	Easy		AGAIN (not start)
540	Single Element in a Sorted Array	Python	O(logn)	O(1)	Medium		OK*
658	Find K Closest Elements	Python	O(logn + k)	O(1)	Medium	good trick, sorting, stack, two pointers,binary search,amazon,fb, google	AGAIN ************** (5) (MUST)
744	Find Smallest Letter Greater Than Target	Python	O(logn)	O(1)	Easy		OK*
852	Peak Index in a Mountain Array	Python	O(logn)	O(1)	Medium	LC 162 Find Peak Element, amazon	OK* (2)
875	Koko Eating Bananas	Python, Java	O(nlogr)	O(1)	Medium	binary search, left boundary, good basic	AGAIN******* (4)(MUST)
894	All Possible Full Binary Trees	Python	O(n * 4^n / n^(3/2))	O(n * 4^n / n^(3/2))	Medium		AGAIN (not start)
911	Online Election	Python	ctor: O(n) query : O(logn)	O(n)	Medium		AGAIN (not start)
981	Time Based Key-Value Store	Python, Java		O(n)	Medium	binary search, dict, treeMap, floorKey, sort, apple, M$, amz, google	AGAIN****** (2)
1011	Capacity To Ship Packages Within D Days	Python, Java			Medium	good trick, binary search, amazon, apple, fb, m$, google	AGAIN****** (3)
1889	Minimum Space Wasted From Packaging	Python			Hard	heap, prefix sum, binary search, amazon	AGAIN (not start)
2009	Minimum Number of Operations to Make Array Continuous	Python			Hard	binary search, sliding window, dequeue, bisect.bisect_right, amazon	AGAIN (not start)

Binary Search Tree

#	Title	Solution	Time	Space	Difficulty	Tag,Note	Status
220	Contains Duplicate III	Python	O(nlogk)	O(k)	Medium		AGAIN (not start)
230	Kth Smallest Element in a BST	Python, Java	O(max(h, k))	O(min(h, k))	Medium	Curated Top 75, good basic BST basic, DFS, BFS, STACK, BST,amazon	OK********* (4)
235	Lowest Common Ancestor of a Binary Search Tree	Python, Java	O(h)	O(1)	Medium	Curated Top 75, BST, recursion, iteration, good basic, LCA, check # 236 Lowest Common Ancestor of a Binary Tree, amazon,fb	OK**************** (but again)(10)
270	Closest Binary Search Tree Value	Python	O(h)	O(1)	Easy	good basic ,🔒, M$, BST, google,fb	OK***** (5)
285	Inorder Successor in BST	Python	O(h)	O(1)	Medium	BST, dfs, good basic, 🔒,amazon,fb	OK** (4)
449	Serialize and Deserialize BST	Python	O(n)	O(h)	Medium	LC 297, dfs, bfs, good trick,serizlize deserizlize, tree, amazon, fb	AGAIN************* (9) (MUST)
450	Delete Node in a BST	Python	O(h)	O(h)	Medium	AGAIN, good trick, BST, tree, dfs	AGAIN****** (6) (but again)
530	Minimum Absolute Difference in BST	Python	O(n)	O(h)	Easy		AGAIN*
700	Search in a Binary Search Tree	Python, Java	O(n)	O(h)	Easy	BST, good basic, recursion, iteration amazon	AGAIN*** (1)
776	Split BST	Python	O(n)	O(h)	Medium	🔒, BST, tree, DFS, trick, basic, AGAIN ,amazon	AGAIN******* (5) (not start***)
783	Minimum Distance Between BST Nodes	Python	O(n)	O(h)	Easy		OK*
426	Convert Binary Search Tree to Sorted Doubly Linked List	Python			Medium	recursion, tree, linked list, good basic UBER, lyft, amazon, google,fb	AGAIN******* (5) (not start)
968	Binary Tree Cameras	Python		O(n)	Hard	amazon, dfs, bst, dp, greedy, m$	AGAIN (not start)
1022	Sum of Root To Leaf Binary Numbers	Python		O(n)	Easy	good basic, LC 257, bst path, dfs, bfs,amazon	AGAIN******** (1) (MUST)
1597	Build Binary Expression Tree From Infix Expression	Python		O(n)	Hard	LC 224, recursive, iteration, stack, amazon, google, bst	AGAIN*** (1)

Breadth-First Search

#	Title	Solution	Time	Space	Difficulty	Tag	Status
102	Binary Tree Level Order Traversal	Python, Java	O(n)	O(n)	Easy	Curated Top 75, GOOD basic, dfs, bfs,UBER, apple,amazon,fb, LC 199	AGAIN******* (8) (must)
107	Binary Tree Level Order Traversal II	Python, Java	O(n)	O(n)	Easy	same as # 102 Binary Tree Level Order Traversal	OK* (3)
103	Binary Tree Zigzag Level Order Traversal	Python	O(n)	O(n)	Medium	amazon,fb	OK
126	Word Ladder II	Python			Hard	complex, trick, dfs, bfs, dfs+bfs, check # 127 Word Ladder,amazon	AGAIN*** (3)
127	Word Ladder	Python, Java	O(n * d)	O(d)	Hard/Medium	good basic, check #126 Word Ladder II, bfs, UBER, amazon, M$, fb	AGAIN************** (9)
130	Surrounded Regions	Python, Java	O(m + n)		Medium	bfs, dfs,union find,good basic, amazon	AGAIN*********** (5)
133	Clone Graph	Python, Java	O(n)	O(n)	Medium	Curated Top 75, good trick, check #138 Copy List with Random Pointer ,graph,dfs,bfs, UBER, google,amazon,fb	AGAIN**************** (10) (MUST)
207	Course Schedule	Python, Java	O(|V| + |E|)	O(|E|)	Medium	Curated Top 75, Topological Sort, backtrack, good trick,dfs, bfs , amazon,fb	AGAIN**************** (13) (MUST)
210	Course Schedule II	Python	O(|V| + |E|)	O(|E|)	Medium	Topological Sort,check # 207 Course Schedule first, dfs, bfs ,amazon ,fb	AGAIN********* (9) (again)
261	Graph Valid Tree	Python, Java	O(|V| + |E|)	O(|V| + |E|)	Medium	Curated Top 75, AGAIN, bfs, dfs, grpah, 🔒, graph,quick union,union find ,google,amazon,fb	AGAIN************* (11)(MUST)
286	Walls and Gates	Python, Java	O(m * n)	O(g)	Medium	🔒, dfs, bfs , good basic, google,amazon,FB	AGAIN******** (7)
310	Minimum Height Trees	Python	O(n)	O(n)	Medium	complex	AGAIN (3) (not start)
433	Minimum Genetic Mutation	Python	O(n * b)	O(b)	Medium	check # 127 Word Ladder, good basic	AGAIN*** (3)
444	Sequence Reconstruction	Python	O(n * s)	O(n)	Medium	🔒 Topological Sort , google, airbnb	AGAIN** (3) (not start)
490	The Maze	Python	O(max(r, c) * w)	O(w)	Medium	basic, bfs, dfs,amazon,fb	AGAIN**** (5)
505	The Maze II	Python	O(max(r, c) * wlogw)	O(w)	Medium	trick,bfs,dfs,AGAIN, google, fb	AGAIN********* (6)
542	01 Matrix	Python	O(m * n)	O(m * n)	Medium	DP, trick,queue	AGAIN*** (3)
666	Path Sum IV	Python	O(n)	O(w)	Medium	🔒 Topological Sort	AGAIN* (3) (not start)
742	Closest Leaf in a Binary Tree	Python, Java	O(n)	O(n)	Medium	AGAIN, Graph, bfs+dfs, search, good trick, amazon	AGAIN********** (6)
743	Network Delay Time	Python, Java	O(n)	O(n)	Medium	Dijlstra, google	AGAIN **** (2)
752	Open the Lock	Python	O(k * n^k + d)	O(k * n^k + d)	Medium	LC 863	AGAIN (not start)
787	Cheapest Flights Within K Stops	Python	O(|E| * log|V|)	O(|E|)	Medium	Dijkstra's algorithm, dfs, bfs, graph, priority queue,amazon, apple, google, airbnb	AGAIN****** (3) (not start)
815	Bus Routes	Python	O(|E| * log|V|)	O(|E|)	Hard	shortest route, graph, bfs,amazon	AGAIN*** (2) (not start)
886	Possible Bipartition	Python	O(|V| + |E|)	O(|V| + |E|)	Medium	check # 785 Is Graph Bipartite?,graph, AGAIN,union find ,fb	AGAIN********** (6)
934	Shortest Bridge	Python	O(n^2)	O(n^2)	Medium	BFS, DFS, complex	AGAIN (not start)
967	Numbers With Same Consecutive Differences	Python	O(2^n)	O(2^n)	Medium	good trick	AGAIN** (3)
675	Cut Off Trees for Golf Event	Python, Java			Hard	tree, BFS, complex, amazon	AGAIN (not start)
864	Shortest Path to Get All Keys	Python			Hard	BFS, amazon	AGAIN (not start)
994	Rotting Oranges	Python, Java			Medium	BFS, dp amazon	OK** (2)
1110	Delete Nodes And Return Forest	Python, Java			Medium	BFS, recursive google	not start
1162	As Far from Land as Possible	Python			Medium	dfs, bfs, good basic, amazon	AGAIN**** (3)
1197	Minimum Knight Moves	Java			Medium	BFS, google	OK (1)
1730	Shortest Path to Get Food	Python			Medium	shortest path, bfs, amazon	OK (2)

Depth-First Search

#	Title	Solution	Time	Space	Difficulty	Tag	Status
112	Path Sum	Python, Scala, Java	O(n)	O(h)	Easy	good concept, amazon, dfs, recursion	AGAIN** (2)
113	Path Sum II	Python, Java	O(n)	O(h)	Medium	dfs, backtrack, LC 112 Path Sum,good basic, amazon, fb	AGAIN****** (6)
199	Binary Tree Right Side View	Python, Java	O(n)	O(h)	Medium	good basic, dfs, bfs,apple,amazon, fb, LC 102	OK*** (6)
200	Number of Islands	Python, Java	O(m * n)	O(m * n)	Medium	Curated Top 75, bfs, dfs,good basic,check # 694, 711 Number of Distinct Islands,apple, Goldman Sachs,google,amazon,fb	OK****** (6)
236	Lowest Common Ancestor of a Binary Tree	Python, Java	O(n)	O(h)	Medium	trick,EPI, LCA,check 235 Lowest Common Ancestor of a Binary Search Tree,apple,amazon,linkedin,fb	AGAIN************* (9)
247	Strobogrammatic Number II	Python	O(n^2 * 5^(n/2))	O(n)	Medium	🔒,check #246 Strobogrammatic Number,good trick, google,fb	AGAIN******** (4)
250	Count Univalue Subtrees	Python	O(n)	O(h)	Medium	🔒	AGAIN (not start)
257	Binary Tree Paths	Python, Java	O(n * h)	O(h)	Easy	good basic,apple, google, amazon, fb, iterative, dfs, LC 112, 113	AGAIN********* (MUST) (7)
332	Reconstruct Itinerary	Python	O(t! / (n1! * n2! * ... nk!))	O(t)	Medium	yelp, google	AGAIN* (3) (not start)
339	Nested List Weight Sum	Python	O(n)	O(h)	Easy	🔒,good basic,bfs, linkedin, fb	AGAIN*** (3)
364	Nested List Weight Sum II	Python	O(n)	O(h)	Medium	🔒, linkedin	AGAIN*
366	Find Leaves of Binary Tree	Python	O(n)	O(h)	Medium	🔒, linkedin	AGAIN*
399	Evaluate Division	Python	O(q * |V|!)	O(e)	Medium	union find, dfs, AGAIN, good trick google, fb	AGAIN*********** (6)
417	Pacific Atlantic Water Flow	Python, Java	O(m * n)	O(m * n)	Medium	Curated Top 75, 2 direction dfs, dfs, matrix, m$,google	AGAIN** (3)
464	Can I Win	Python	O(n!)	O(n)	Medium	DP, linkedin	AGAIN* (not start)
491	Increasing Subsequences	Python			Medium	trick,dfs,dfs good trick, dp,yahoo ,fb	AGAIN***** (2)
515	Find Largest Value in Each Tree Row	Python	O(n)	O(h)	Medium	good basic,linkedin	AGAIN* (3)
529	Minesweeper	Python			Medium	dfs, bfs, good basic, dfs, bfs,amazon	AGAIN**** (4)
547	Friend Circles	Python	O(n^2)	O(n)	Medium	graph, Union Find,dfs, bfs,check # 733 Flood Fill,bloomberg ,fb	AGAIN*** (3) (not start)
547	Number of Provinces	Python	O(n^2)	O(n)	Medium	dfs, bfs, graph, union find, good basic, apple, amazon	AGAIN************ (2) (MUST)
582	Kill Process	Python	O(n)	O(n)	Medium	🔒, DFS, BFS,good basic ,bloomberg, amazon	AGAIN***** (4)
638	Shopping Offers	Python	O(n * 2^n)	O(n)	Medium	google	AGAIN (not start*)
690	Employee Importance	Python	O(n)	O(h)	Easy	DFS, BFS,good basic ,UBER	OK** (3)
694	Number of Distinct Islands	Python	O(m * n)	O(m * n)	Medium	#200, #711 Number of Islands good pattern, dfs, 🔒, compare with # 200 Number of Islands, amazon	AGAIN******** (5)
695	Max Area of Island	Python, Java	O(m * n)	O(m * n)	Medium	amazon, microsoft, linkedin, basic	AGAIN* (3)
711	Number of Distinct Islands II	Python, Java			Hard	complex, dfs, check # 200, 694 Number of Distinct Islands,amazon	OK*** (3)
721	Accounts Merge	Python	O(nlogn)	O(n)	Medium	dfs, Disjoint Set Union (DSU), Union Find,path compression, complex, fb, google, amazon, m$, apple, twitter	AGAIN******* (4) (not start)
733	Flood Fill	Python	O(m * n)	O(m * n)	Easy	fb, amazon, good basic	OK**** (5)
737	Sentence Similarity II	Python, Java	O(m * n)	O(m * n)	Medium	dfs, union find, dis-joint, google	AGAIN**** (2)
756	Pyramid Transition Matrix	Python	O(a^b)	O(a^b)	Medium		AGAIN (3) (not start)
785	Is Graph Bipartite?	Python	O(|V| + |E|)	O(|V|)	Medium	check #886 Possible Bipartition,AGAIN,graph,dfs,fb, amazon	AGAIN*********** (6)
797	All Paths From Source to Target	Python	O(p + r * n)	O(n)	Medium	graph, dfs classics, good basic	AGAIN** (3)
802	Find Eventual Safe States	Python, Java	O(|V| + |E|)	O(|V|)	Medium	Topological Sort, dfs, bfd, graph, google	AGAIN (4) (not start)
841	Keys and Rooms	Python	O(n!)	O(n)	Medium	google, bfs, dfs, good basic	AGAIN*** (3)
851	Loud and Rich	Python	O(q + r)	O(q + r)	Medium	amazon,defaultdict, good basic	AGAIN*** (3)
1087	Brace Explansion	Java	O(q + r)	O(q + r)	Medium	dfs, backtrack, good trick, google	AGAIN*** (1)
1192	Critical Connections in a Network	Python	O(q + r)	O(q + r)	Hard	Tarjan's algorithm,dfs, graph, fb, amazon	AGAIN** (1) (not start)
1644	Lowest Common Ancestor of a Binary Tree II	Python	O(q + r)	O(q + r)	Medium	LCA, LC 236, m$, fb	AGAIN (not start)
1650	Lowest Common Ancestor of a Binary Tree III	Python	O(q + r)	O(q + r)	Medium	parent node, good trick, dfs, set, dict, LCA, LC 236, amazon, google, spotify m$, fb	AGAIN******** (2) (MUST)
1676	Lowest Common Ancestor of a Binary Tree IV	Python	O(q + r)	O(q + r)	Medium	LCA, LC 236, a m$	AGAIN (not start)

Backtracking

#	Title	Solution	Time	Space	Difficulty	Tag, Note	Status
017	Letter Combinations of a Phone Number	Python, Java	O(n * 4^n)	O(n)	Medium	dfs, backtrack, good trick,UBER, amazon, google, fb	AGAIN************* (10) (MUST)
022	Generate Parentheses	Python, Java	O(4^n / n^(3/2))	O(n)	Medium	LC 20, good basic, stack, backtrac, Divide and Conquer, amazon	OK**************** (5) (but again, MUST)
037	Sudoku Solver	Python	O(4^n / n^(3/2))	O(n)	Hard	backtrack, google, amazon, uber	AGAIN(not start)
039	Combination Sum	Python, Java	O(k * n^k)	O(k)	Medium	backtrack,dfs,good basic,UBER, airbnb, amazon,fb	OK********* (7) (but again)
040	Combination Sum II	Python, Java	O(k * C(n, k))	O(k)	Medium	similar as #39 Combination Sum	AGAIN*** (3) (again)
046	Permutations	Python, Java	O(n * n!)	O(n)	Medium	backtrack, LC #77, 78, good concept, M$, linkedin, amazon,fb	OK*********** (4) (AGAIN, MUST)
047	Permutations II	Python	O(n * n!)	O(n)	Medium	similar as #46 Permutations, apple, amazon, m$	AGAIN (not start)
052	N-Queens II	Python	O(n^2)	O(1)	Hard	matrix, backtrack	AGAIN
077	Combinations	Python, Java	O(O(k * C(n, k)))	O(k)	Medium	good trick, backtrack	OK************* (5) (but again, MUST)
079	Word Search	Python, Java	O(m * n * l)	O(l)	Medium	LC 212, Curated Top 75, back tracking,good trick,dfs,bloomberg, M$, amazon,fb	AGAIN*************** (10) (MUST)
093	Restore IP Addresses	Python	O(1)	O(1)	Medium	good trick,recursive,IP address,amazon	AGAIN****** (4)
078	Subsets	Python, Java	O(n * 2^n)	O(1)	Medium	good concept , similar as, dfs, back track, #90 Subsets II , UBER, amazon, fb	AGAIN**************** (7) (MUST again!!!)
090	Subsets II	Python, Java	O(n * 2^n)	O(1)	Medium	good basic,check # 078 Subsets, dfs, backtrack, fb	AGAIN********* (6)
131	Palindrome Partitioning	Python, Java	O(n^2) ~ O(2^n)	O(n^2)	Medium	dfs, backtracking, good trick,amazon	AGAIN*********** (6) (MUST)
139	Word Break	Python, Java	O(n * l^2)	O(n)	Medium	Curated Top 75, backtracking, dfs, dp,trick,UBER,yahoo,amazon, google,fb	AGAIN**************** (7) (MUST)
140	Word Break II	Python	O(n * l^2)	O(n)	Hard	good basic, LC 078, backtracking, dfs, dp,trick, amazon	AGAIN******** (2)
212	Word Search II	Python, Java	O(m * n * l)	O(l)	Hard	Curated Top 75, LC 208, LC 079, hashset, hashmap, trie, backtrack with trie, amazon, fb, apple, twitter, uber, google, indeed	AGAIN********** (3)
216	Combination Sum III	Python	O(k * C(n, k))	O(k)	Medium		AGAIN*
254	Factor Combinations	Python	O(nlogn)	O(logn)	Medium	🔒	AGAIN (not start)
267	Palindrome Permutation II	Python	O(n * n!)	O(n)	Medium	🔒	AGAIN (not start*)
294	Flip Game II	Python	O(n + c^2)	O(c)	Medium	🔒, DP, Hash	AGAIN*
320	Generalized Abbreviation	Python	O(n * 2^n)	O(n)	Medium	🔒	AGAIN (not start*)
489	Robot Room Cleaner	Python	O(n * 2^n)	O(n)	Hard	🔒, google, amazon, fb, m$	AGAIN (not start)
526	Beautiful Arrangement	Python	O(n!)	O(n)	Medium		AGAIN (not start*)
676	Implement Magic Dictionary	Python	O(n)	O(d)	Medium	Trie, DFS	AGAIN (not start)
698	Partition to K Equal Sum Subsets	Python	O(n * 2^n)	O(2^n)	Medium	DFS, DP, Memoization, similar as #416 Partition Equal Subset Sum, #473 Matchsticks to Square	AGAIN (not start*)
718	Maximum Length of Repeated Subarray	Python	O(m * n)	O(min(m, n))	Medium	DP, Hash, Binary Search	AGAIN (not start*)
784	Letter Case Permutation	Python	O(n * 2^n)	O(1)	Medium	dfs, recursion, good trick,fb	AGAIN****** (3)
980	Unique Paths III	Python	O(n * 2^n)	O(1)	Hard	backtrack, dfs, amazon, google,fb	AGAIN**** (1)
1219	Path with Maximum Gold	Java	O(n * 2^n)	O(1)	Medium	backtrack, dfs, bfs, google	AGAIN**** (1)

Dynamic Programming

#	Title	Solution	Time	Space	Difficulty	Tag	Status
053	Maximum Subarray	Python, Java	O(n)	O(1)	Medium	Curated Top 75, brute force, divied & conquer, DP,dp basic,amazon,fb	AGAIN************* (6)
062	Unique Paths	Python, Java	O(m * n)	O(m + n)	Medium	Curated Top 75, dp, basic, fb, google, apple, amazon, m$	AGAIN**** (2)
063	Unique Paths II	Python	O(m * n)	O(m + n)	Medium	trick, check #062 Unique Paths, amazon	AGAIN*
064	Minimum Path Sum	Python	O(m * n)	O(m + n)	Medium	DP, amazon	AGAIN*
070	Climbing Stairs	Python, Java	O(logn)	O(1)	Easy	Curated Top 75, Matrix Exponentiation, DP, recursion, apple	OK* (2)
072	Edit Distance	Python	O(logn)	O(1)	Hard	top 100 like, DP, recursion, apple, google, amazon	again (not start)
091	Decode Ways	Python, Java	O(n)	O(1)	Medium	Curated Top 75, good basic,dp, M$, UBER, amazon,fb	AGAIN************ (4)
096	Unique Binary Search Trees	Python	O(n)	O(1)	Medium	dp, Math, fb, amazon	AGAIN* (2) (not start)
120	Triangle	Python	O(m * n)	O(n)	Medium	amazon	AGAIN* (not start) (2)
123	Best Time to Buy and Sell Stock III	Python	O(m * n)	O(n)	Hard	amazon, uber, apple	AGAIN* (not start) (1)
152	Maximum Product Subarray	Python, Java	O(n)	O(1)	Medium	dp, brute force, Curated Top 75, google, amazon, linkedin	AGAIN***** (1)
198	House Robber	Python, Java	O(n)	O(1)	Easy	Curated Top 75, dp basic,amazon	AGAIN*** (2)
213	House Robber II	Python, Java	O(n)	O(1)	Medium	brute force, recursion, dp, Curated Top 75, google, amazon	OK**** (3) (but again)
221	Maximal Square	Python, Java	O(n^2)	O(n)	Medium	EPI, dp, amazon,fb, google	AGAIN** (3) (not start)
256	Paint House	Python	O(n)	O(1)	Medium	🔒	AGAIN (not start)
276	Paint Fence	Python	O(n)	O(1)	Easy	🔒	AGAIN
279	Perfect Squares	Python	O(n * sqrt(n))	O(n)	Medium	dp, bfs, trick, Hash, google	AGAIN*
303	Range Sum Query - Immutable	Python	ctor: O(n), lookup: O(1)	O(n)	Easy		OK
304	Range Sum Query 2D - Immutable	Python	ctor: O(m * n), lookup: O(1)	O(m * n)	Medium	dp,trick,fb	AGAIN******* (5)
309	Best Time to Buy and Sell Stock with Cooldown	Python	O(n)	O(1)	Medium		AGAIN (not start)
322	Coin Change	Python, Java	O(n * k)	O(k)	Medium	Curated Top 75, bfs, dfs, dp, grab, google, fb, apple, amazon	AGAIN************* (6) (MUST)
351	Android Unlock Patterns	Python	O(9^2 * 2^9)	O(9 * 2^9)	Medium	🔒 Backtracking, DP, google	AGAIN (not start)
357	Count Numbers with Unique Digits	Python	O(n)	O(1)	Medium	Backtracking, Math	AGAIN (not start)
361	Bomb Enemy	Python	O(m * n)	O(m * n)	Medium	🔒 , google	AGAIN* (2)
363	Max Sum of Rectangle No Larger Than K	Java	O(m * n)	O(m * n)	Medium	dp, pre-sum. google	AGAIN* (not start)
368	Largest Divisible Subset	Python	O(n^2)	O(n)	Medium		AGAIN (not start)
375	Guess Number Higher or Lower II	Python	O(n^2)	O(n^2)	Medium		AGAIN (not start)
377	Combination Sum IV	Python	O(nlogn + n * t)	O(t)	Medium	Curated Top 75, AGAIN, dp basic ,fb	AGAIN******** (3)
403	Frog Jump	Python	O(nlogn + n * t)	O(t)	Hard	dfs, bfs, Memoization dfs, dp, amazon, tik-tok, m$	AGAIN (not start)
416	Partition Equal Subset Sum	Python	O(n * s)	O(s)	Medium	dp, fb, google, apple, amazon	AGAIN** (not start) (1)
418	Sentence Screen Fitting	Python	O(r + n * c)	O(n)	Medium	🔒, dp, google	AGAIN (not start)
467	Unique Substrings in Wraparound String	Python	O(n)	O(1)	Medium		AGAIN (not start)
471	Encode String with Shortest Length	Python	O(n^3) on average	O(n^2)	Medium	🔒, dp, google	AGAIN (not start)
472	Concatenated Words	Python	O(n * l^2)	O(n * l)	Hard	good trick,trie+dfs, dp, dfs,amazon	AGAIN******** (3)
474	Ones and Zeroes	Python	O(s * m * n)	O(m * n)	Medium	dp, google	AGAIN (not start)
486	Predict the Winner	Python	O(n^2)	O(n)	Medium	good basic,dp, google	AGAIN*****
509	Fibonacci Number	Python	O(logn)	O(1)	Easy	variant of Climbing Stairs, Matrix Exponentiation, Spotify	OK**
516	Longest Palindromic Subsequence	Python	O(n^2)	O(n)	Medium	good DP basic,dp, UBER, amazon	AGAIN******
562	Longest Line of Consecutive One in Matrix	Python	O(m * n)	O(n)	Medium	🔒, dp, google	AGAIN (not start)
552	Student Attendance Record II	Java	O(m * n)	O(n)	Medium	dp, google	AGAIN (not start)
576	Out of Boundary Paths	Python	O(N * m * n)	O(m * n)	Medium	dp, bfs	AGAIN (not start)
583	Delete Operation for Two Strings	Python	O(m * n)	O(n)	Medium	dp, google	AGAIN (not start)
650	2 Keys Keyboard	Python	O(sqrt(n))	O(1)	Medium	dp,dp basic ,M$	AGAIN*
673	Number of Longest Increasing Subsequence	Python	O(n^2)	O(n)	Medium	dp,good trick, fb	AGAIN********** (3)
688	Knight Probability in Chessboard	Python	O(k * n^2)	O(n^2)	Medium	dp,dp basic,AGAIN, M$,Goldman Sachs, google, fb, amazon	AGAIN********* (5)
712	Minimum ASCII Delete Sum for Two Strings	Python	O(m * n)	O(n)	Medium	dp	AGAIN (not start)
714	Best Time to Buy and Sell Stock with Transaction Fee	Python	O(n)	O(1)	Medium	AGAIN, good basic, dp, greedy,fb	AGAIN******** (3)
727	Minimum Window Subsequence	Java	O(n)	O(1)	Hard	dp,google	AGAIN (not start)
740	Delete and Earn	Python	O(n)	O(1)	Medium		AGAIN (not start)
746	Min Cost Climbing Stairs	Python	O(n)	O(1)	Easy	good dp basic,dp, amazon	AGAIN**** (2)
750	Number Of Corner Rectangles	Python	O(n * m^2)	O(n * m)	Medium	dp,fb	AGAIN (3) (not start)
764	Largest Plus Sign	Python	O(n^2)	O(n^2)	Medium	dp, complex ,fb	AGAIN (not start) (2)
788	Rotated Digits	Python	O(logn)	O(logn)	Easy	Brute Force, Memoization	OK*
790	Domino and Tromino Tiling	Python	O(logn)	O(1)	Medium	Matrix Exponentiation, dp, google	AGAIN (not start)
799	Champagne Tower	Python	O(n^2)	O(n)	Medium	dp	AGAIN
801	Minimum Swaps To Make Sequences Increasing	Python, Java	O(n)	O(1)	Medium	dp, amazon, fb	AGAIN***** (not start) (2)
808	Soup Servings	Python	O(1)	O(1)	Medium		AGAIN (not start)
813	Largest Sum of Averages	Python	O(k * n^2)	O(n)	Medium	dp, google	AGAIN (not start)
823	Binary Trees With Factors	Python	O(n^2)	O(n)	Medium	dp	AGAIN (not start)
837	New 21 Game	Python, Java	O(n)	O(n)	Medium	dp, trick, math, google	AGAIN (2) (not start)
838	Push Dominoes	Python	O(n)	O(n)	Medium	dp,two pointers,google	AGAIN (not start)
877	Stone Game	Python	O(n^2)	O(n)	Medium	variant of Predict the Winner	AGAIN (not start)
926	Flip String to Monotone Increasing	Python	O(n)	O(1)	Medium	good trick, dp, prefix sum, google, amazon	AGAIN********** (4)
931	Minimum Falling Path Sum	Python	O(n^2)	O(1)	Medium	google, goldman sachs	AGAIN (not start)
935	Knight Dialer	Python	O(logn)	O(1)	Medium	Matrix Exponentiation, google, M$, fb	AGAIN (not start)
518	Coin Change 2	Python	O(logn)	O(1)	Medium	dp basic,good trick,check # 322 Coin Change,dp, google, fb	AGAIN******** (4)
494	Target Sum	Python	O(logn)	O(1)	Medium	dp basic, dp, fb	AGAIN***** (1)
1000	Minimum Cost to Merge Stones	Python	O(logn)	O(1)	Medium	dp, amazon	AGAIN (not start)
1048	Longest String Chain	Java	O(logn)	O(1)	Medium	dp, google	AGAIN (not start)
1143	Longest Common Subsequence	Python	O(logn)	O(1)	Medium	Curated Top 75, Memoization, dp, amazon, karat	AGAIN*** (2) (not start)
1312	Minimum Insertion Steps to Make a String Palindrome	Python	O(logn)	O(1)	Hard	dp, amazon	AGAIN (not start)
1335	Minimum Difficulty of a Job Schedule	Python	O(logn)	O(1)	Medium	dp, amazon	not start
1799	Maximize Score After N Operations	Python	O(logn)	O(1)	Medium	dp, amazon	not start
3196	Maximize Total Cost of Alternating Subarrays	Java	O(logn)	O(1)	Medium	dp, LC weekly	Again

Greedy

#	Title	Solution	Time	Space	Difficulty	Tag, Note	Status
011	Container With Most Water	Python, Java	O(n)	O(1)	Medium	Curated Top 75, good basics, two pointers,fb, amazon	OK*** (3) (but again !!)
045	Jump Game II	Python, Java	O(n)	O(1)	Medium	Greedy, google, apple, fb, tesla, amazon	AGAIN******** (4) (MUST)
055	Jump Game	Python, Java	O(n)	O(1)	Medium	Curated Top 75, good trick, backtrack, Greedy, DP, amazon	AGAIN******** (8)
084	Largest Rectangle in Histogram	Python, Java	O(n)	O(1)	Hard	top 100 like, brute force, divide and conquer, good trick, stack, LC 085, amazon, fb	AGAIN**** (1)
122	Best Time to Buy and Sell Stock II	Python	O(n)	O(1)	Easy	compare with #309 Best Time to Buy and Sell Stock with Cooldown , #714 Best Time to Buy and Sell Stock with Transaction Fee, amazon	OK* (2)
134	Gas Station	Python, Java	O(n)	O(1)	Medium	trick, greedy,amazon	AGAIN****** (5)
135	Candy	Python, Java	O(n)	O(1)	Hard	LC 123, greedy, boundary, array, amazon	AGAIN (2)
218	The Skyline Problem	Python	O(n)	O(1)	Hard	brute force, swipe line, Priority Queue, Union Find, Divide-and-Conquer, apple, microsoft	AGAIN (not start)
330	Patching Array	Python	O(n)	O(1)	Hard	array, greedy,amazon	AGAIN (not start)
358	Rearrange String k Distance Apart	Python	O(n)	O(1)	Medium	heap, greedy, check LC 767	not start
376	Wiggle Subsequence	Python	O(n)	O(1)	Medium		AGAIN*
392	Is Subsequence	Python	O(n)	O(1)	Medium	basics	OK*
397	Integer Replacement	Python	O(n)	O(1)	Medium	Recursion basics, Math	OK*
402	Remove K Digits	Python	O(n)	O(n)	Medium	LintCode	AGAIN (not start*)
435	Non-overlapping Intervals	Python, Java	O(nlogn)	O(1)	Medium	Curated Top 75, good trick, 2 pointers, inverval, similiar as #452 Minimum Number of Arrows to Burst Balloons, Line Sweep, google	AGAIN************ (8)
452	Minimum Number of Arrows to Burst Balloons	Python, Java	O(nlogn)	O(1)	Medium	sort, compare i-1 element, overlap, good basic	AGAIN***** (3)
455	Assign Cookies	Python	O(nlogn)	O(1)	Easy		OK*
621	Task Scheduler	Python, Java	O(n)	O(1)	Medium	trick,array,greedy, fb, amazon	AGAIN****** (7)
646	Maximum Length of Pair Chain	Python	O(nlogn)	O(1)	Medium	good trick, greedy, similar as #435 Non-overlapping Intervals, Line Sweep, amazon	OK********** (5) (but again)
649	Dota2 Senate	Python	O(n)	O(n)	Medium	complex question	AGAIN (not start*)
659	Split Array into Consecutive Subsequences	Python, Java	O(n)	O(1)	Medium	map, pq, greedy, dp, google	AGAIN***** (1)
738	Monotone Increasing Digits	Python	O(1)	O(1)	Medium	good trick, greedy, string, trick, Amazon	AGAIN********* (4)
763	Partition Labels	Python	O(n)	O(n)	Medium	greedy,trick,dict,index,amazon	AGAIN******* (5) (AGAIN)
767	Reorganize String	Python	O(n)	O(1)	Medium	greedy,counter,good trick,amazon,fb	AGAIN**************** (7) (MUST)
861	Score After Flipping Matrix	Python	O(r * c)	O(1)	Medium		AGAIN (not start*)
870	Advantage Shuffle	Python	O(nlogn)	O(n)	Medium		OK*
881	Boats to Save People	Python	O(nlogn)	O(n)	Medium	good trick	OK*
948	Bag of Tokens	Python	O(nlogn)	O(1)	Medium	Two Pointers	AGAIN (not start)
962	Maximum Width Ramp	Python	O(n)	O(n)	Medium	Descending Stack	AGAIN (not start)
1024	Video Stitching	Python			Medium	Spotify	AGAIN (not start)
1102	Path With Maximum Minimum Value	Python			Medium	dfs, dp , greedy, amazon	not start
319	Bulb Switcher	Python			Medium		not start
672	Bulb Switcher II	Python			Medium		not start
1375	Bulb Switcher III	Python			Medium	queue, array, gra*	AGAIN*** (1)
1648	Sell Diminishing-Valued Colored Balls	Python			Medium	trick, sorting, heapq, binary search, greedy, amazon	AGAIN*** (2) (not start)
1710	Maximum Units on a Truck	Python			Easy	sort, greedy, amazon	AGAIN******** (3) (MUST)

Graph

#	Title	Solution	Time	Space	Difficulty	Status	Note
269	Alien Dictionary	Python, Java			Hard	Curated Top 75, dfs, bfs, topology sort, fb, google, m$, airbnb, uber, amazon	AGAIN******** (4) (not start)
323	Number of Connected Components in an Undirected Graph	Python , Java	O(n)	O(n)	Medium	Curated Top 75, LC 547, 🔒 , dfs, Union Find, graph, linkedin, amazon, fb, google, m$, linkedin	AGAIN************** (5) (MUST)
959	Regions Cut By Slashes	Python	O(n^2)	O(n^2)	Medium	Union Find
1135	Connecting Cities With Minimum Cost	Python	O(n^2)	O(n^2)	Medium	union find, Kruskal, prime, graph, amazon	AGAIN (not start)

Geometry

#	Title	Solution	Time	Space	Difficulty	Status	Note
892	Surface Area of 3D Shapes	C++ Python	O(n^2)	O(1)	Easy
0587	Erect the Fence	Python	O(nlogn)	O(n)	Hard		complex, Convex Hull, Monotone Chain , amazon
0892	Surface Area of 3D Shapes

Design

#	Title	Solution	Time	Space	Difficulty	Tag	Note
146	LRU Cache	Python, Java	O(1)	O(k)	Medium	good basic, LRU, Least Recently Used, Double Linkedlist, LinkedHashMap, OrderedDict, priority queue, amazon	AGAIN****** (4)
225	Implement Stack using Queues	Python, Java	push: O(n), pop: O(1), top: O(1)	O(n)	Easy		AGAIN** (1)
284	Peeking Iterator	C++ Python	O(1)	O(1)	Medium	good basic, google, apple	not start*** (1)
348	Design Tic-Tac-Toe	Python	O(1)	O(n^2)	Medium	matrix, array, 🔒, google, fb, amazon	AGAIN****** (5)
353	Design Snake Game	Python, Java	O(1)	O(s)	Medium	good trick, Deque, 🔒, amazon, google, square, m$	AGAIN*** (1)
355	Design Twitter	Python, Java	O(klogu)	O(t + f)	Medium	good trick, data structure, heapq, defaultdict, amazon	OK******** (6)
0362	Design Hit Counter	C++ Python	O(1), amortized	O(k)	Medium	🔒	Deque
0379	Design Phone Directory	C++ Python	O(1)	O(n)	Medium	🔒
0380	Insert Delete GetRandom O(1)	Python	O(1)	O(n)	Medium/Hard	set, list, dict, amazon,fb	OK* (5)
0381	Insert Delete GetRandom O(1) - Duplicates allowed	C++ Python	O(1)	O(n)	Hard
432	All O`one Data Structure	C++ Python	O(1)	O(n)	Hard
460	LFU Cache	Python	O(1)	O(k)	Hard	Least Frequently Used (LFU) cache, complex, amazon	AGAIN*** (2) (not start)
535	Encode and Decode TinyURL	Python	O(1)	O(n)	Medium	hash map, "duplicated", "hash map collision" ,UBER, amazon,google ,fb, design	OK**** (5)
588	Design In-Memory File System	C++ Python	ls: O(l + klogk) mkdir: O(l) addContentToFile: O(l + c) readContentFromFile: O(l + c)	O(n + s)	Hard	dict, a bit complex basic, 🔒, shoptify, amazon	AGAIN******* (3)
0604	Design Compressed String Iterator	C++ Python	O(1)	O(1)	Easy	🔒
622	Design Circular qeque	Python			Medium	🔒
631	Design Excel Sum Formula	C++ Python	set: O((r * c)^2) get: O(1) sum: O((r * c)^2)	O(r * c)	Hard	🔒
635	Design Log Storage System	C++ Python	put: O(1) retrieve: O(n + dlogd)	O(n)	Medium	🔒
641	Design Circular Deque	Python			Medium	🔒
642	Design Search Autocomplete System	Python	O(p^2)	O(p * t + s)	Hard	🔒, amazon	AGAIN**** (not start) (5)
705	Design HashSet	Python			Easy
706	Design HashMap	Python			Easy
0715	Range Module	C++ Python	add: O(n) remove: O(n) query: O(logn)	O(n)	Hard
716	Max Stack	Python	push: O(logn) pop: O(logn) popMax: O(logn) top: O(1) peekMax: O(1)	O(n)	Easy	basic, stack, amazon	again*
745	Prefix and Suffix Search	Python	ctor: O(w * l^2) search : O(p + s)	O(t)	Hard	Trie, fb	Not start* (1) (good basic)
900	RLE Iterator	C++ Python	O(n)	O(1)	Medium
1146	Snapshot Array	Java	set: O(1) get: O(logn)	O(n)	Medium	treeMap, binary search, google	AGAIN*** (1)
1166	Design File System	C++ Python	create: O(n) get: O(n)	O(n)	Medium	🔒, design, google, airbnb, amazon	AGAIN***** (1)
1172	Dinner Plate Stacks	C++ Python	push: O(logn) pop: O(1), amortized popAtStack: (logn)	O(n * c)	Hard
1206	Design Skiplist	C++ Python	O(logn), on average	O(n)	Hard
1236	Web Crawler	C++ Python	O(|V| + |E|)	O(|V|)	Medium	🔒, BFS, DFS
1244	Design A Leaderboard	C++ Python	ctor: O(1) add: O(1) top: O(n) reset: O(1)	O(n)	Medium
1268	Search Suggestions System	Python	ctor: O(n * l) suggest: O(l^2)	O(t)	Medium	good basic, array, heap, sort, Trie, amazon	AGAIN******** (3)
1286	Iterator for Combination	C++ Python	O(k)	O(k)	Medium	Stack
1429	First Unique Number	Python	O(k)	O(k)	Medium	good trick, heap, amazon	AGAIN*** (not start)
1472	Design Browser History	Python	O(k)	O(k)	Medium		AGAIN
1500	Design a File Sharing System	Python	O(k)	O(k)	Medium	design, hashmap	AGAIN*** (not start)
1603	Design Parking System	Python	O(k)	O(k)	Easy	design, amazon , paypal	OK
1628	Design an Expression Tree With Evaluate Function	Python	O(k)	O(k)	Medium	complex, design, amazon	AGAIN*** (not start)
1656	Design an Ordered Stream	Python	O(k)	O(k)	Easy
1670	Design Front Middle Back Queue	Python	O(k)	O(k)	Medium	good basic, Design, amazon	AGAIN** (1) (not start)
1756	Design Recent Used Queue	Python	O(k)	O(k)	Medium	design, queue	AGAIN
1797	Design Authentication Manager	Python	O(k)	O(k)	Medium	design, hashmap	AGAIN
1976	Number of Ways to Arrive at Destination	Python	O(k)	O(k)	Medium

Simulation

#	Title	Solution	Time	Space	Difficulty	Status	Note
874	Walking Robot Simulation	C++ Python	O(n + k)	O(k)	Easy

Concurrency

#	Title	Solution	Time	Space	Difficulty	Tag	Note
1114	Print in Order	C++ Python	O(n)	O(1)	Easy
1115	Print FooBar Alternately	C++ Python	O(n)	O(1)	Medium
1116	Print Zero Even Odd	C++ Python	O(n)	O(1)	Medium
1117	Building H2O	C++ Python	O(n)	O(1)	Hard
1188	Design Bounded Blocking Queue	C++ Python	O(n)	O(1)	Medium	🔒
1195	Fizz Buzz Multithreaded	C++ Python	O(n)	O(1)	Medium
1226	The Dining Philosophers	C++ Python	O(n)	O(1)	Medium
1242	Web Crawler Multithreaded	C++ Python	O(|V| + |E|)	O(|V|)	Medium	🔒
1279	Traffic Light Controlled Intersection	C++ Python	O(n)	O(1)	Easy	🔒

SQL

#	Title	Solution	Time	Space	Difficulty	Tag, Note	Status
0175	Combine Two Tables	MySQL	O(m + n)	O(m + n)	Easy		OK (4)
0176	Second Highest Salary	MySQL	O(n)	O(1)	Easy	amazon,fb	OK (4)
0177	Nth Highest Salary	MySQL	O(n^2)	O(n)	Medium	MySQL PROCEDURE, SQL FAQ	AGAIN*** (3)
0178	Rank Scores	MySQL	O(n^2)	O(n)	Medium	trick, SQL var, ank, select same table, rank	AGAIN****** (7)
0180	Consecutive Numbers	MySQL	O(n)	O(n)	Medium	SQL var, join same table multi times	OK* (3)
0181	Employees Earning More Than Their Managers	MySQL	O(n^2)	O(1)	Easy		OK
0182	Duplicate Emails	MySQL	O(n^2)	O(n)	Easy		OK
0183	Customers Who Never Order	MySQL	O(n^2)	O(1)	Easy	left join	OK** (3) (but again)
0184	Department Highest Salary	MySQL	O(n^2)	O(n)	Medium	trick, where, good basic	OK***** (but again) (6)
0185	Department Top Three Salaries	MySQL	O(n^2)	O(n)	Hard	similiar as # 0177 Nth Highest Salary, good trick, top N in group	AGAIN******** (5)
0196	Delete Duplicate Emails	MySQL	O(n^2)	O(n)	Easy, delete command		AGAIN (2)
0197	Rising Temperature	MySQL	O(n^2)	O(n)	Easy		OK
0262	Trips and Users	MySQL	O((t * u) + tlogt)	O(t)	Hard	case condisions	OK* (2)
0511	Game Play Analysis I	MySQL	O(n)	O(n)	Easy	🔒	OK
0512	Game Play Analysis II	MySQL	O(n)	O(n)	Easy	🔒, 2 col where good trick	AGAIN**(4)
0534	Game Play Analysis III	MySQL	O(nlogn)	O(n)	Medium	🔒, SQL var, tricky, window func	AGAIN******** (6)
0550	Game Play Analysis IV	MySQL	O(n)	O(n)	Medium	case, fraction, join, good basic, where min condition	OK*********** (7) (but again)
569	Median Employee Salary	MySQL	O(n)	O(n)	Medium	medium number	AGAIN* (not start)
571	Find Median Given Frequency of Numbers	MySQL	O(n)	O(n)	Medium	medium number	AGAIN** (not start) (2)
577	Employee Bonus	MySQL	O(n)	O(n)	Easy		OK
578	Get Highest Answer Rate Question	MySQL			Medium	fb, case condition	OK* (3)
579	Find Cumulative Salary of an Employee	MySQL			Hard	window function, amazon, good trick	OK***** (5)(but again)
580	Count Student Number in Departments	MySQL			Medium		OK*** (2) (but again)
595	Big Countries	MySQL			Easy		OK
597	Friend Requests I: Overall Acceptance Rate	MySQL			Medium	select distinct, ifnull,fb	OK*** (7)
601	Human Traffic of Stadium	MySQL			Hard	consecutive nums	OK* (1)
602	Friend Requests II: Who Has Most Friend?	MySQL			Medium	union all, fb	OK*** (4)
603	Consecutive Available Seats	MySQL			Medium	consecutive nums, good basic, cross join	OK***** (3)(but again)
618	Students Report By Geography	MySQL			Hard	sql var	AGAIN (not start)
620	Not Boring Movies	MySQL			Easy		OK
626	Exchange Seats	MySQL			Medium		AGAIN (not start)
627	Swap Salary	MySQL			Easy	UPDATE with CASE, good basic	OK* (1) (but again)
1045	Customers Who Bought All Products	MySQL	O(n + k)	O(n + k)	Medium	🔒, where with count	OK*** (4)
1050	Actors and Directors Who Cooperated At Least Three Times	MySQL	O(n)	O(n)	Easy	🔒	OK
1068	Product Sales Analysis I	MySQL	O(m + n)	O(m + n)	Easy	🔒	OK
1069	Product Sales Analysis II	MySQL	O(n)	O(n)	Easy	🔒	OK
1070	Product Sales Analysis III	MySQL	O(n)	O(n)	Medium	🔒	OK
1075	Project Employees I	MySQL	O(m + n)	O(m + n)	Easy	🔒	OK
1076	Project Employees II	MySQL	O(n)	O(n)	Easy	🔒	OK (2)
1077	Project Employees III	MySQL	O((m + n)^2)	O(m + n)	Medium	🔒, tricky where	OK***** (2) (but again)
1082	Sales Analysis I	MySQL	O(n)	O(n)	Easy	🔒, group by only 1 col, good basic	OK***** (5) (again)
1083	Sales Analysis II	MySQL	O(m + n)	O(m + n)	Easy	🔒, not in	OK*** (3) (again)
1084	Sales Analysis III	MySQL	O(m + n)	O(m + n)	Easy	🔒	OK** (2)
1097	Game Play Analysis V	MySQL	O(n^2)	O(n)	Hard	🔒, good basic,retention, case condition, retention, data_add, left join	AGAIN******* (8)
1098	Unpopular Books	MySQL	O(m + n)	O(n)	Medium	🔒, where not in	OK* (2)
1107	New Users Daily Count	MySQL	O(n)	O(n)	Medium	🔒	OK*
1112	Highest Grade For Each Student	MySQL	O(nlogn)	O(n)	Medium	🔒, good basic	OK** (4)
1113	Reported Posts	MySQL	O(n)	O(n)	Easy	🔒	OK
1126	Active Businesses	MySQL	O(n)	O(n)	Medium	🔒, good basic	AGAIN*** (3)
1127	User Purchase Platform	MySQL	O(n)	O(n)	Hard	🔒, complex, union all, case	OK**** (6) (but again)
1132	Reported Posts II	MySQL	O(m + n)	O(n)	Medium	🔒	OK*** (4)
1141	User Activity for the Past 30 Days I	MySQL	O(n)	O(n)	Easy	🔒	AGAIN* (2)
1142	User Activity for the Past 30 Days II	MySQL	O(n)	O(n)	Easy	🔒	AGAIN* (1)
1148	Article Views I	MySQL	O(nlogn)	O(n)	Easy	🔒	OK
1149	Article Views II	MySQL	O(nlogn)	O(n)	Medium	🔒	OK*
1158	Market Analysis I	MySQL	O(m + n)	O(m + n)	Medium	🔒, case when ... is not null	AGAIN**** (5)
1159	Market Analysis II	MySQL	O(m + n)	O(m + n)	Hard	🔒, good trick	AGAIN** (3)
1164	Product Price at a Given Date	MySQL	O(mlogn)	O(m)	Medium	🔒, good trick, union, case, not in a period	AGAIN******** (2)
1173	Immediate Food Delivery I	MySQL	O(n)	O(1)	Easy	🔒	OK
1174	Immediate Food Delivery II	MySQL	O(n)	O(m)	Medium	🔒	OK**** (3)
1179	Reformat Department Table	MySQL	O(n)	O(n)	Easy	🔒	OK
1193	Monthly Transactions I	MySQL	O(n)	O(n)	Medium	🔒	OK (2)
1194	Tournament Winners	MySQL	O(m + n + nlogn)	O(m + n)	Hard	🔒	AGAIN*
1204	Last Person to Fit in the Elevator	MySQL	O(nlogn)	O(n)	Medium	🔒, cumsum, window func, good basic	OK****** (3)
1205	Monthly Transactions II	MySQL	O(n)	O(n)	Medium	🔒, good trick, union all, case conditions	OK*** (3) (but again)
1211	Queries Quality and Percentage	MySQL	O(n)	O(n)	Easy		OK
1212	Team Scores in Football Tournament	MySQL	O(nlogn)	O(n)	Medium	good trick	OK** (2)
1225	Report Contiguous Dates	MySQL	O(nlogn)	O(n)	Hard	🔒	AGAIN* (2) (not start)
1241	Number of Comments per Post	MySQL	O(n)	O(n)	Easy	🔒, good basic	OK**** (5) (but again)
1251	Average Selling Price	MySQL	O(n)	O(n)	Easy	🔒, good basic	OK*** (but again) (3)
1264	Page Recommendations	MySQL	O(m + n)	O(m)	Medium	🔒	OK** (2)
1270	All People Report to the Given Manager	MySQL	O(n)	O(n)	Medium	🔒, good basic	OK* (3)
1280	Students and Examinations	MySQL	O((m * n) * log(m * n))	O(m * n)	Easy	🔒	OK (1)
1285	Find the Start and End Number of Continuous Ranges	MySQL	O(n)	O(n)	Medium	🔒, variable	AGAIN (not start) (1)
1294	Weather Type in Each Country	MySQL	O(m + n)	O(n)	Easy	🔒	OK (1)
1303	Find the Team Size	MySQL	O(n)	O(n)	Easy	🔒	ok (1)
1308	Running Total for Different Genders	MySQL	O(nlogn)	O(n)	Medium	🔒, good basic, union all, window func	OK**** (3)
1321	Restaurant Growth	MySQL	O(n^2)	O(n)	Medium	🔒, good basic, trick	AGAIN***** (3)
1322	Ads Performance	MySQL	O(nlogn)	O(n)	Easy	🔒, good basic	OK******* (3) (but again)
1327	List the Products Ordered in a Period	MySQL	O(n)	O(n)	Easy	🔒	OK
1336	Number of Transactions per Visit	MySQL	O(m + n)	O(m + n)	Medium	🔒, complex, variable	AGAIN (not start)
1341	Movie Rating	MySQL	O(nlogn)	O(n)	Medium	🔒	AGAIN (not start)
1350	Students With Invalid Departments	MySQL	O(n)	O(n)	Easy	🔒	OK
1355	Activity Participants	MySQL	O(n)	O(n)	Medium	🔒	OK
1364	Number of Trusted Contacts of a Customer	MySQL	O(n + m + l + nlogn)	O(n + m + l)	Medium	🔒	OK
1369	Get the Second Most Recent Activity	MySQL	O(nlogn)	O(n)	Hard	🔒, union, having	OK** (2) (but again)
1378	Replace Employee ID With The Unique Identifier	MySQL	O(n)	O(n)	Easy	🔒	OK
1384	Total Sales Amount by Year	MySQL	O(nlogn)	O(n)	Hard	🔒, complex	OK*
1393	Capital Gain/Loss	MySQL	O(n)	O(n)	Medium	🔒	OK
1398	Customers Who Bought Products A and B but Not C	MySQL	O(m + n)	O(m + n)	Medium	🔒	OK
1407	Top Travellers	MySQL	O(m + nlogn)	O(m + n)	Easy	🔒, ifnull	OK*
1412	Find the Quiet Students in All Exams	MySQL	O(m + nlogn)	O(m + n)	Hard	🔒, good trick, not in, inner join on multiple cols	OK**** (3) (but again)
1421	NPV Queries	MySQL	O(n)	O(n)	Medium	🔒	OK
1435	Create a Session Bar Chart	MySQL	O(n)	O(1)	Easy	🔒	OK
1440	Evaluate Boolean Expression	MySQL	O(n)	O(n)	Medium	🔒	OK
1445	Apples & Oranges	MySQL	O(n)	O(n)	Medium	🔒	OK
1454	Active Users	MySQL	O(nlogn)	O(n)	Medium	🔒, good basic, window func	AGAIN*** (2)
1459	Rectangles Area	MySQL	O(n^2)	O(n^2)	Medium	🔒	OK*
1468	Calculate Salaries	MySQL	O(m + n)	O(m + n)	Easy	🔒	OK*
1479	Sales by Day of the Week	MySQL	O(m + n)	O(n)	Hard	🔒, case, DAYOFWEEK	OK
1484	Group Sold Products By The Date	MySQL	O(nlogn)	O(n)	Easy	🔒, GROUP_CONCAT	OK*
1495	Friendly Movies Streamed Last Month	MySQL	O(n)	O(n)	Easy	🔒	OK
1501	Countries You Can Safely Invest In	MySQL	O(n)	O(n)	Medium	🔒,good trick, group avg, global avg, SUBSTRING, inner join on or conditions	OK****** (4)
1511	Customer Order Frequency	MySQL	O(n)	O(n)	Easy	🔒, having, like	OK* (1)
1517	Find Users With Valid E-Mails	MySQL	O(n)	O(n)	Easy	🔒, regular expression	AGAIN
1527	Patients With a Condition	MySQL	O(n)	O(n)	Easy	🔒	OK
1532	The Most Recent Three Orders	MySQL	O(nlogn)	O(n)	Medium	🔒, window func, good basic	OK** (2) (but again)
1543	Fix Product Name Format	MySQL	O(nlogn)	O(n)	Easy	🔒, trim	OK
1549	The Most Recent Orders for Each Product	MySQL	O(nlogn)	O(n)	Medium	🔒, max day per id, good basic	OK* (again)
1555	Bank Account Summary	MySQL	O(m + n)	O(m + n)	Medium	🔒, union all, good trick	OK**** (2)(but again)
1565	Unique Orders and Customers Per Month	MySQL	O(n)	O(n)	Easy	🔒	OK
1571	Warehouse Manager	MySQL	O(n)	O(n)	Medium	🔒	OK* (1)
1581	Customer Who Visited but Did Not Make Any Transactions	MySQL	O(n)	O(n)	Easy	🔒, left join, good basic	OK** (2)(but again)
1587	Bank Account Summary II	MySQL	O(m + n)	O(m + n)	Easy	🔒	OK
1596	The Most Frequently Ordered Products for Each Customer	MySQL	O(n)	O(n)	Medium	🔒, good trick, rank() ORDER BY count, get max in each group	OK********* (3)(but again)
1607	Sellers With No Sales	MySQL	O(nlogm)	O(n + m)	Medium	🔒	OK
1613	Find the Missing IDs	MySQL	O(n^2)	O(n)	Medium	🔒, RECURSIVE CTE, good trick	AGAIN*** (2)
1623	All Valid Triplets That Can Represent a Country	MySQL	O(n^3)	O(n^3)	Easy	🔒, <>	AGAIN* (not start)
1633	Percentage of Users Attended a Contest	MySQL	O(m + nlogn)	O(n)	Easy	🔒	OK
1635	Hopper Company Queries I	MySQL	O(d + r + tlogt)	O(d + r + t)	Hard	🔒, complex	AGAIN (not start)
1645	Hopper Company Queries II	MySQL	O(d + r + tlogt)	O(d + r + t)	Hard	🔒, complex	AGAIN (not start)
1651	Hopper Company Queries III	MySQL	O(d + r + tlogt)	O(d + r + t)	Hard	🔒, complex	AGAIN (not start)
1661	Average Time of Process per Machine	MySQL	O(n)	O(n)	Easy	🔒, good basic	OK* (1)
1667	Fix Names in a Table	MySQL	O(nlogn)	O(n)	Easy	🔒, concat, sub-string	OK (1)
1677	Product's Worth Over Invoices	MySQL	O(nlogn)	O(n)	Easy	🔒	OK
1683	Invalid Tweets	MySQL	O(n)	O(n)	Easy	🔒	OK
1693	Daily Leads and Partners	MySQL	O(n)	O(n)	Easy	🔒	OK
1699	Number of Calls Between Two Persons	MySQL	O(n)	O(n)	Medium	🔒, cte, union all	OK* (1)(but again)
1709	Biggest Window Between Visits	MySQL	O(nlogn)	O(n)	Medium	🔒, window func, datediff, coalesce, good trick	AGAIN***** (2)
1715	Count Apples and Oranges	MySQL	O(n)	O(n)	Medium	🔒, ifnull	OK (2)
1729	Find Followers Count	MySQL	O(nlogn)	O(n)	Easy	🔒	OK
1731	The Number of Employees Which Report to Each Employee	MySQL	O(nlogn)	O(n)	Easy	🔒	OK
1741	Find Total Time Spent by Each Employee	MySQL	O(nlogn)	O(n)	Easy	🔒	OK
1747	Leetflex Banned Accounts	MySQL	O(n^2)	O(n)	Medium	🔒	OK (2) (but again)
1757	Recyclable and Low Fat Products	MySQL	O(n)	O(n)	Easy	🔒	OK
1767	Find the Subtasks That Did Not Execute	MySQL	O(n * c)	O(n * c)	Hard	🔒, RECURSIVE CTE	AGAIN (not start)
1777	Product's Price for Each Store	MySQL	O(n)	O(n)	Easy	🔒	OK (2)
1783	Grand Slam Titles	MySQL	O(n)	O(n)	Medium	🔒, union all, good basic	OK* (1) (but again)
1789	Primary Department for Each Employee	MySQL	O(n)	O(n)	Easy	🔒, where id in	OK (1) (but again)
1795	Rearrange Products Table	MySQL	O(n)	O(n)	Easy	🔒, union all, good basic	OK****** (2)
1809	Ad-Free Sessions	MySQL	O(n)	O(n)	Easy	🔒, left join where col is null	OK**** (3)(but again)
1811	Find Interview Candidates	MySQL	O(nlogn)	O(n)	Medium	🔒, union all, cte, where, good basic	AGAIN* (2)
1821	Find Customers With Positive Revenue this Year	MySQL	O(n)	O(n)	Easy	🔒	OK
1831	Maximum Transaction Each Day	MySQL	O(nlogn)	O(n)	Medium	🔒, rank(), where on 2 attr, good basic	AGAIN**** (2)
1841	League Statistics	MySQL	O(nlogn)	O(n)	Medium	🔒, case, complex	AGAIN* (2)
1843	Suspicious Bank Accounts	MySQL	O(nlogn)	O(n)	Medium	🔒, good trick, window func	OK** (2) (but AGAIN)
1853	Convert Date Format	MySQL	O(n)	O(n)	Easy	🔒	OK
1867	Orders With Maximum Quantity Above Average	MySQL	O(n)	O(n)	Easy	🔒	OK (1)
1873	Calculate Special Bonus	MySQL	O(n)	O(n)	Easy	🔒, left, odd num	OK (2)
1875	Group Employees of the Same Salary	MySQL	O(nlogn)	O(n)	Medium	🔒, complex, good basic	AGAIN* (1)
1890	The Latest Login in 2020	MySQL	O(n)	O(n)	Easy	🔒	OK
1892	Page Recommendations II	MySQL	O(n * m)	O(n * m) Hard	🔒	AGAIN*** (1) (not start)
1907	Count Salary Categories	MySQL	O(n)	O(n)	Medium	🔒, union all	OK* (1)
1917	Leetcodify Friends Recommendations	MySQL	O(n^2)	O(n^2)	Hard	🔒, NOT EXISTS	AGAIN*** (2)
1919	Leetcodify Similar Friends	MySQL	O(n * l)	O(n * l)	Hard	🔒	AGAIN* (not start)
1934	Confirmation Rate	MySQL	O(n + m)	O(n + m)	Medium	🔒, good basic	OK* (1) (but again)
1939	Users That Actively Request Confirmation Messages	MySQL	O(nlogn)	O(n)	Easy	🔒, TIMESTAMPDIFF, good basic	OK*** (2) (but again)
1949	Strong Friendship	MySQL	O(n^3)	O(n^2)	Medium	🔒	good basic, where in 2 cols
1951	All the Pairs With the Maximum Number of Common Followers	MySQL	O(n^3)	O(n^2)	Medium	🔒, good trick	AGAIN* (2)
1965	Employees With Missing Information	MySQL	O(nlogn)	O(n)	Easy	🔒, good basic	OK* (1) (but again)
1972	First and Last Call On the Same Day	MySQL	O(n)	O(n)	Hard	🔒, rank, union all, amazon	AGAIN** (2)
1978	Employees Whose Manager Left the Company	MySQL	O(nlogn)	O(n)	Easy	🔒	OK (1)
1988	Find Cutoff Score for Each School	MySQL	O(n * m)	O(n * m)	Medium	🔒, no ref	AGAIN (not start)
1990	Count the Number of Experiments	MySQL	O(n)	O(n)	Easy	🔒, cross join	AGAIN* (1)
2004	The Number of Seniors and Juniors to Join the Company	MySQL	O(nlogn)	O(n)	Hard	🔒, not ref	AGAIN (not start)

Shell Script

#	Title	Solution	Time	Space	Difficulty	Tag	Note
0192	Word Frequency	Shell	O(n)	O(k)	Medium
0193	Valid Phone Numbers	Shell	O(n)	O(1)	Easy
0194	Transpose File	Shell	O(n^2)	O(n^2)	Medium
0195	Tenth Line	Shell	O(n)	O(1)	Easy