Leetcode🫡

Table of Contents

1. Mapping & Arrays
2. Stack
3. Two Pointer
4. Binary Search
5. Matrix
6. Sliding Window
7. Linked List
8. Trees
9. Tries
10. Heap & Priority Queue
11. Backtracking
12. Graphs
13. Dynamic Programing

Mapping & Arrays

Problem	Solution	Difficulty	Hint
Two Sum	Python	Easy	Check for diff and map value to index
Roman To Integer	Python C++	Easy	Map Roman to Int
Find Words That Can Be Formed by Characters	Python	Easy	Freq Count
Remove Element	Python	Easy	Keep track of num of elements not in nums
Pascal Triangle	Python	Easy	Check prev values to find curr
Unique Email Addresses	Python	Easy	Use set
Find the Difference of Two Arrays	Python	Easy	create ans = [[], []] and loop thru nums1 and nums2
Find the Index of First Occurence in String	Python	Easy	Slicing
Search Insert Position	Python	Easy	Loop through and compare
Contains Duplicate	Python C++	Easy	Use set to check
Contains Duplicate II	Python	Easy	Map mp[num]: i
Happy Number	Python	Easy	Keep track of visited numbers
Isomorphic Strings	Python	Easy	Map curr s_char with t_char and keep track of used t_chars
Valid Anagram	Python C++	Easy	Map with occurences
Zigzag Conversion	Python	Medium	Create list of empty strings size of rows -> check if index at end of list
Search In Rotated Sorted Array	Python	Medium	Binary Search with first and second half of array
Max Number of K-Sum Pairs	Python	Medium	Map difference to count and check if num in dict
Group Anagams	Python C++	Medium	Use sorted and hash with list
Minimum Number of Frogs Croaking	Python	Medium	Update new frog when arrive at 'c' -> map char with count
Longest Consecutive Sequence	Python C++	Medium	Keep track of previous index
Prodcut of Array Except Self	Python	Medium	create left and right list [1] * len(nums), get product of left and right in nums except [0] and [-1], match index and multiply
Divide Array Into Arrays With Max Difference	Python	Medium	One pass
Top K Frequent Elements	Python	Medium	Bubble sort
Longest Consecutive Sequence	Python	Medium	Convert nums to a set and check if start of sequence -> end of sequence
Find the Winner of an Array Game	Python	Medium	Compare 0th index continuously
Last Moment Before All Ants Fall Out of a Plank	Python	Medium	Find max ant from falling
Minimum Amount of Time to Collect Garbage	Python	Medium	Calculate (total time cost) + (trash count)
Count Nice Pairs in an Array	Python	Medium	Map num-rev : count
Candy	Python	Hard	Two pass and map index: candy_ct

Stack

Problem	Solution	Difficulty	Hint
Valid Parenthesses	Python C++	Easy	Pop from stack if valid
Evaluate Reverse Polish Notation	Python	Medium	Pop 2x and append
Min Stack	Python	Medium	Append and tuple and compare with top of stack
Simplify Stack	Python	Medium	Split string by "/" then one pass with stack
Daily Temperatures	Python	Medium	Use stack and append with (val, index) then sliding window
Car Fleet	Python	Medium	Create list of pos and speed then reverse list to get distance (target - pos)/speed
Insert Delete GetRandom O(1)	Python	Medium	Swap with top of stack and map {val: index}

Two Pointer

Problem	Solution	Difficulty	Hint
Palindrome Number	Python C++	Easy	Cast to string and 2 pointer
Longest Common Prefix	Python	Easy	Create helper function to compare with first string in list
Remove Duplicates from Sorted Array	Python	Easy	Recursion or 2 pointer
Container with Most Water	Python	Medium	Two pointer
Two Sum II - Input Array is Sorted	Python	Medium	Two pointer
3Sum	Python	Medium	Two pointer and create a visited set with tuples
Remove Duplicates from Sorted Array II	Python	Medium	Replace duplicate with long int
Trapping Rain Water	Python	Hard	2 Pointer and keep track of left and right max

Binary Search

Problem	Solution	Difficulty	Hint
Binary Search	Python	Easy	Create left,right, and middle pointer
Search a 2D Matrix	Python C++	Medium	Find row by comparing 0th and -1th index -> regular binary search on row
Find Minimum in Rotated Sorted Array	Python	Medium	If right pointer greater than curr pointer move window down
Find First and Last Position of Element in Sorted Array	Python	Medium	Two pass binary search
Search in Rotated Sorted Array	Python	Medium	Check left -> mid and mid -> bounds to see if sorted
Koko Eating Bananas	Python	Medium	Binary search and keep track of amount of bannas ate
Time Based Key-Value Store	Python	Medium	Map key to list of lists and binary search(left = 0, right = length of key))

Matrix

Problem	Solution	Difficulty	Hint
Matrix Diagonal Sum	Python C++	Easy	Two pointer
Special Positions in a Binary Matrix	Python	Easy	Map r,c to counts of '1'
Valid Sudoku	Python C++	Medium	Map with R, C, and 3x3
Spiral Matrix	Python	Medium	Keep track of L,R,T,B
Set Matrix Zeros	Python	Medium	Create set for rows and cols with zeros
Search a 2D Matrix	Python	Medium	Binary Search
Game of Life	Python	Medium	Use set to store indexes for zeros and ones
Rotate Matrix 90 Degrees	Python	Medium	Take transpose then go through range n//2 then swap [i][j], [j][i] <-> [i][n-j], [i][j]

Sliding Window

Problem	Solution	Difficulty	Hint
Best Time to Buy and Sell Stock	Python	Easy	Keep track of highest
Maximum Average Subarray l	Python	Easy	Find max sum -> return max_sum/k
Best Time to Buy and Sell Stock II	Python	Medium	Add to profit if profit[i] < profit[i+1]
Longest Substring Without Repeating Characters	Python C++	Medium	Slding window
Permutation in String	Python	Medium	Sliding window
Merge Intervals	Python	Medium	Sort by start time -> sliding window
Maximum Number of Vowels in a Substring of Given Length	Python	Medium	Find count from 0 -> k then sliding window k -> len(s)
Minimum Size Subarray Sum	Python	Medium	Sliding window
Find Longest Subarray By Sum	Python	Medium	increase window size until sum > target then minimize left side of window until > target
Minimum Window Substring	Python	Hard	Monotonic decreasing queue, make sure right-left index is < k, append to res when right >= k-1

Linked List

Problem	Solution	Difficulty	Hint
Reverse Linked List	Python	Easy	Keep Track of prev curr and next
Merge Two Sorted Lists	Python	Easy	Traverse and connect head to remaning list1 or list 2
Remove Duplicates From Sorted Linked List	Python	Easy	Keep track of next node and check if curr.next
Linked List Cycle	Python	Easy	Use slow and fast pointer to check for overlap
Add Two Numbers	Python	Medium	Go left to right and keep track of carry
Remove Nth Node From End of List	Python	Medium	Create dummy node and 2 pointer
Swap Nodes In Pairs	Python	Medium	Keep track of curr.next and curr.next.next
Reverse Linked List II	Python	Medium	3 pass and use multiple pointers
Copy List With Random Pointer	Python	Medium	Map old node to new node
Partition List	Python	Medium	One pass with 2 dummy nodes
Reorder List	Python	Medium	Find middle using 2 ptr, reverse list end -> middle, merge
LRU Cache	Python	Medium	Use a map and deque
Remove K from List	Python	Medium	check for curr.next.next and return head if head.val != k
Convert Sorted Linked List to Binary Search Tree	Python	Medium	Create list of vals and dfs helper function
Split Linked List in Parts	Python	Medium	Get length of list and append to ans when part size hit
Merge k Sorted Lists	Python	Hard	Create helper function to merge 2 lists and merge first list with rest
Reverse Nodes in k-Group	Python	Hard	Use stack to keep track of k nodes

Trees

Problem	Solution	Difficulty	Hint
Binary Tree Preorder Traversal	Python	Easy	DFS
Sum of Left Leaves	Python	Easy	DFS
Binary Tree Postorder Traversal	Python	Easy	Recrusive DFS or use stack and BFS iterativly and return reversed list
Count Complete Tree Nodes	Python	Easy	BFS and add by length of queue
Bianry Tree Paths	Python	Easy	DFS and checkk if not root.left and not root.right
Path Sum	Python	Easy	DFS and keep track of current sum
Mode in BST	Python	Easy	DFS and map frequency
Convert Sorted Array To BST	Python	Easy	Use recursion and keep track of middle to find sub node
Maximum Depth of Binary Tree	Python	Easy	recursive dfs and return 1 + max of left and right subtree
Minimum Depth of Binary Tree	Python	Easy	DFS and update node count in paramenter
Invert Binary Tree	Python	Easy	Swap nodes the recurse
Diameter of Binary Tree	Python	Easy	DFS and check for max(left+right+1,max(left,right))
Balanced Binary Tree	Python	Easy	Create bool variable for ans, dfs and check if max(left,right) - min(left,right) > 1, change ans
Same Tree	Python	Easy	dfs and append to 2 lists
Subtree of Another Tree	Python	Easy	dfs until root.val == subRoot.val and use helper function for same tree
Lowest Common Ancestor of BST	Python	Medium	DFS until split (root > p.val and root < q.val (vise versa))
Sum Root to Leaf Numbers	Python	Medium	DFS and keep track of int with str
Pseudo-Palindromic Paths in a Binary Tree	Python	Medium	DFS -> keep cts of values -> check if max odd value is <= 1
Path Sum ll	Python	Medium	DFS with stack and check when sum(stack) = target
Path Sum lll	Python	Medium	Brute force and get sum of every root
Amount of Time for Binary Tree to Be Infected	Python	Medium	Convert to undirected graph then dfs
Maximum Difference Between Node and Ancestor	Python	Medium	Return min and max pairs to calculate diff
Symetric Tree	Python	Medium	DFS with invert recursive call
Binary Tree Level Order Traversal	Python	Medium	BFS
Find Largest Value in Each Tree Row	Python	Medium	BFS
Kth Largest Sum Binary Tree	Python	Medium	BFS
Binary Tree ZigZag Level Order Traversal	Python	Medium	BFS and use list slcing to reverse
Maximum Level Sum of Binary Tree	Python	Medium	DFS and check if not root.left and not root.right
Count Nodes Equal to Average of Subtree	Python	Medium	DFS and return tuple of previous edges and sum to avoid brute force
Validate Binary Search Tree	Python	Medium	DFS and keep track of curr min and max
Binary Tree Right Side View	Python	Medium	Create left and right deque and append from end of deque
Count Good Noes in Binary Tree	Python	Medium	DFS and keep track of curr max
Kth Smallest Element in BST	Python	Medium	In order Traversal -> return k-1
Maximum Binary Tree	Python	Medium	Get max of nums -> -> dfs with left and right subarray
Print Binary Tree	Python	Medium	Get height -> create matrix -> dfs
Binary Tree Maximum Sum	Python	Hard	DFS and compare left and right recursive calls with max(0,left or right) to get edge case of root itself being max
Serialize and Deserialize Binary Tree	Python	Hard	DFS to turn tree to str then use refernce incremnter to loop through split data

Tries

Problem	Solution	Difficulty	Hint
Implement Tries (Prefix Tree)	Python	Medium	Create node class of hashmap to map each char to a unique path
Design Add and Search Words Data Structures	Python	Medium	Create node class of hashmap and dfs for search (explore all paths if char == '.')

Heap & Priority Queue

Problem	Solution	Difficulty	Hint
Kth Largest Element in a Stream	Python	Easy	Keep min heap size of k and return root
Seat Reservation Manager	Python	Medium	Min heap
Eliminate Maximum Number of Monsters	Python	Medium	Use min heap and push when monster will arrive to city by dist[i]/spped[i]
Last Stone Weight	Python	Medium	Max heap
K Closest Points to Origin	Python	Medium	Min heap with tuple
K Largest Element in Array	Python	Medium	Max heap or sorted

Backtracking

Problem	Solution	Difficulty	Hint
Subsets	Python	Medium	DFS use stack to take care of 'dont include' descison
Subsets ll	Python	Medium	Sort -> dfs and do not include duplicates in choices
Generate Parentheses	Python	Medium	DFS -> compare open with n -> compare open with closed
Combination Sum	Python	Medium	DFS until index > lenth or sum(stack) == target or sum(stack) > target
Combination Sum ll	Python	Medium	Sort list -> dfs backtrack -> skip over duplicates
Word Search	Python	Medium	DFS and keep track of visited and replace board with original values once depth reached
Permutations	Python	Medium	Keep track of choices with boolean list size of nums
Combinations	Python	Medium	Backtrack
Beautiful Arrangement	Python	Medium	Backtrack
Non Decreasing Subsequences	Python	Medium	Backtrack and store perms in a set
Palidrome Partitioning	Python	Medium	Dfs and check for palidrome
Sudoku Solver	Python	Hard	Backtrack with checking if next empty spot is valid
N-Queens	Python	Hard	Backtrack and keep tract of pos and neg diags (r+c) and (r-c)

Graphs

Problem	Solution	Difficulty	Hint
Flood Fill	Python	Easy	DFS
Number of Islands	Python	Medium	DFS and change '1' to random char
Max Area of Island	Python	Medium	DFS use list to store counts to avoid counter resetting
Pacific Atlantic Water Flow	Python	Medium	Find paths for atlantic and pacific and store in a set
Surrounded Regions	Python	Medium	Find invalid regions using dfs and store in a set -> find valid replacements going through orginal board
Rotting Oranges	Python	Medium	BFS and check neighbors
Course Schedule	Python	Medium	DFS topological sort
Course Schedule ll	Python	Medium	DFS topological sort and check for courses not in prerequisites
Clone Graph	Python	Medium	Map node to new node -> dfs
Redundant Connections	Python	Medium	DFS
Network Delay Time	Python	Medium	Dijkstra algorithm
Cheapest Flights Within K Stops	Python	Medium	Dijkstra algorithm using stops as priority
Path with Minimum Effort	Python	Medium	Dijkstra + memo
Restore the Array From Adjacent Pairs	Python	Medium	Create adj list -> find vertex with outdegree of 1 -> create ans iterativly
Minimum Genetic Mutation	Python	Medium	BFS similar to Word Ladder
Word Ladder	Python	Hard	BFS and get possible paths by using temp char to replace current word
Longest Increasing Path in a Matrix	Python	Hard	DFS and memoization
Design Graph With Shortest Path Calculator	Python	Hard	Dijkstra algorithm

Dynamic Programming

Problem	Solution	Difficulty	Hint
Climbing Stairs	Python	Easy	Bottom up and find sum of previous 2 steps
Min Cost Climbing Stairs	Python	Easy	Bottom up and compare minimum of current cost and next 2 steps
Array Max Consective Sum	Python	Medium	Keep track of curr max and global max and use kadones algorithm