Explanation inside the code.

Key functions added in the end of each question.

For question details, please go to Leetcode

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1. Breadth First Search

   • Easy 101 Symmetric Tree -- [re = list(reversed(li))]
   • Easy 107 Binary Tree Level Order Traversal II --[results[::-1]]
   • Easy 559 Maximum Depth of N-ary Tree
   • Medium 993 Cousins in Binary Tree
   • Medium 515 Find Largest Value in Each Tree Row --[float('-inf')]

2. Depth First Search

   Iteration VS recursive.

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   • Pre-order traversal
   • In-order traversal
   • Post-order traversal
   • Medium 200 Number of Islands

3. Hashing

   The use of collections.Counter() and dictionary.

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   • Easy 884 Uncommon Words from Two Sentences -- [collections.Counter()]
   • Easy 387 First Unique Character in a String -- [collections.Counter()]--[float('inf')]--[index = string.find('c')]
   • Easy 1512. Number of Good Pairs [math.comb(n,k)]
   • Medium 347 Top K Frequent Elements -- [counts = sorted(dic.values())]
   • Easy 1 Two Sum -- [for i,item in enumerate(nums)]
   • Medium 3 Longest Substring Without Repeating Characters
   • Medium 146. LRU Cache -- [collections.OrderedDict()]--[dict.popitem(last=False)]

4. Sliding window

   When to use sliding window? Question like:

   • "finding the substring that meet certain criterion."

   Right pointer loop through list, while substr does not meet criterion, do something that impact the criterion move left pointer

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   • Medium 438 Find All Anagrams in a String
   • Medium 209. Minimum Size Subarray Sum
   • Medium 904. Fruit Into Baskets

5. Linked List

   • Medium 2 Add Two Numbers -- [carry = carry//10]
   • Medium 19 Remove Nth Node From End of List
   • Easy 21 Merge Two Sorted Lists

6. Tree

   • Medium 105 Construct Binary Tree from Preorder and Inorder Traversal -- recursive

7. Two Pointers

   When to use two pointers? Question like:

   • "handle the list/string IN PLACE, I want to do something when elements meet certain criterion"

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   • Medium 763 Partition Labels
   • Easy 27. Remove Element
   • Easy 283. Move Zeroes
   • Easy 844. Backspace String Compare
   • Easy 977. Squares of a Sorted Array

8. Array-binary search

   • Easy 937. Reorder Data in Log Files -- [list.sort(reverse=False, key=(key1,key2))]
   • Easy 704. Binary Search -- [Binary Search]
   • Easy 69. Sqrt(x) -- [Binary Search]
   • Easy 367. Valid Perfect Square -- [Binary Search]