Array

• Two Sum - https://leetcode.com/problems/two-sum/
• Best Time to Buy and Sell Stock - https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
• Contains Duplicate - https://leetcode.com/problems/contains-duplicate/
• Product of Array Except Self - https://leetcode.com/problems/product-of-array-except-self/
• Maximum Subarray - https://leetcode.com/problems/maximum-subarray/
• Maximum Product Subarray - https://leetcode.com/problems/maximum-product-subarray/
• Find Minimum in Rotated Sorted Array - https://leetcode.com/problems/find-minimum-in-rotated-sorted-array/
• Search in Rotated Sorted Array - https://leetcode.com/problems/search-in-rotated-sorted-array/
• 3Sum - https://leetcode.com/problems/3sum/
• Container With Most Water - https://leetcode.com/problems/container-with-most-water/

Binary

• Sum of Two Integers - https://leetcode.com/problems/sum-of-two-integers/
• Number of 1 Bits - https://leetcode.com/problems/number-of-1-bits/
• Counting Bits - https://leetcode.com/problems/counting-bits/
• Missing Number - https://leetcode.com/problems/missing-number/
• Reverse Bits - https://leetcode.com/problems/reverse-bits/

Dynamic Programming

• Climbing Stairs - https://leetcode.com/problems/climbing-stairs/
• Coin Change - https://leetcode.com/problems/coin-change/
• Longest Increasing Subsequence - https://leetcode.com/problems/longest-increasing-subsequence/
• Longest Common Subsequence -
• Word Break Problem - https://leetcode.com/problems/word-break/
• Combination Sum - https://leetcode.com/problems/combination-sum-iv/
• House Robber - https://leetcode.com/problems/house-robber/
• House Robber II - https://leetcode.com/problems/house-robber-ii/
• Decode Ways - https://leetcode.com/problems/decode-ways/
• Unique Paths - https://leetcode.com/problems/unique-paths/
• Jump Game - https://leetcode.com/problems/jump-game/

Graph

• Clone Graph - https://leetcode.com/problems/clone-graph/
• Course Schedule - https://leetcode.com/problems/course-schedule/
• Pacific Atlantic Water Flow - https://leetcode.com/problems/pacific-atlantic-water-flow/
• Number of Islands - https://leetcode.com/problems/number-of-islands/
• Longest Consecutive Sequence - https://leetcode.com/problems/longest-consecutive-sequence/
• Alien Dictionary (Leetcode Premium) - https://leetcode.com/problems/alien-dictionary/
• Graph Valid Tree (Leetcode Premium) - https://leetcode.com/problems/graph-valid-tree/
• Number of Connected Components in an Undirected Graph (Leetcode Premium) - https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/

Interval

• Insert Interval - https://leetcode.com/problems/insert-interval/
• Merge Intervals - https://leetcode.com/problems/merge-intervals/
• Non-overlapping Intervals - https://leetcode.com/problems/non-overlapping-intervals/
• Meeting Rooms (Leetcode Premium) - https://leetcode.com/problems/meeting-rooms/
• Meeting Rooms II (Leetcode Premium) - https://leetcode.com/problems/meeting-rooms-ii/

Linked List

• Reverse a Linked List - https://leetcode.com/problems/reverse-linked-list/
• Detect Cycle in a Linked List - https://leetcode.com/problems/linked-list-cycle/
• Merge Two Sorted Lists - https://leetcode.com/problems/merge-two-sorted-lists/
• Merge K Sorted Lists - https://leetcode.com/problems/merge-k-sorted-lists/
• Remove Nth Node From End Of List - https://leetcode.com/problems/remove-nth-node-from-end-of-list/
• Reorder List - https://leetcode.com/problems/reorder-list/

Matrix

• Set Matrix Zeroes - https://leetcode.com/problems/set-matrix-zeroes/
• Spiral Matrix - https://leetcode.com/problems/spiral-matrix/
• Rotate Image - https://leetcode.com/problems/rotate-image/
• Word Search - https://leetcode.com/problems/word-search/

String

• Longest Substring Without Repeating Characters - https://leetcode.com/problems/longest-substring-without-repeating-characters/
• Longest Repeating Character Replacement - https://leetcode.com/problems/longest-repeating-character-replacement/
• Minimum Window Substring - https://leetcode.com/problems/minimum-window-substring/
• Valid Anagram - https://leetcode.com/problems/valid-anagram/
• Group Anagrams - https://leetcode.com/problems/group-anagrams/
• Valid Parentheses - https://leetcode.com/problems/valid-parentheses/
• Valid Palindrome - https://leetcode.com/problems/valid-palindrome/
• Longest Palindromic Substring - https://leetcode.com/problems/longest-palindromic-substring/
• Palindromic Substrings - https://leetcode.com/problems/palindromic-substrings/
• Encode and Decode Strings (Leetcode Premium) - https://leetcode.com/problems/encode-and-decode-strings/

Tree

• Maximum Depth of Binary Tree - https://leetcode.com/problems/maximum-depth-of-binary-tree/
• Same Tree - https://leetcode.com/problems/same-tree/
• Invert/Flip Binary Tree - https://leetcode.com/problems/invert-binary-tree/
• Binary Tree Maximum Path Sum - https://leetcode.com/problems/binary-tree-maximum-path-sum/
• Binary Tree Level Order Traversal - https://leetcode.com/problems/binary-tree-level-order-traversal/
• Serialize and Deserialize Binary Tree - https://leetcode.com/problems/serialize-and-deserialize-binary-tree/
• Subtree of Another Tree - https://leetcode.com/problems/subtree-of-another-tree/
• Construct Binary Tree from Preorder and Inorder Traversal - https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
• Validate Binary Search Tree - https://leetcode.com/problems/validate-binary-search-tree/
• Kth Smallest Element in a BST - https://leetcode.com/problems/kth-smallest-element-in-a-bst/
• Lowest Common Ancestor of BST - https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
• Implement Trie (Prefix Tree) - https://leetcode.com/problems/implement-trie-prefix-tree/
• Add and Search Word - https://leetcode.com/problems/add-and-search-word-data-structure-design/
• Word Search II - https://leetcode.com/problems/word-search-ii/

Heap

• Merge K Sorted Lists - https://leetcode.com/problems/merge-k-sorted-lists/
• Top K Frequent Elements - https://leetcode.com/problems/top-k-frequent-elements/
• Find Median from Data Stream - https://leetcode.com/problems/find-median-from-data-stream/

Tips: If input array is sorted then

• Binary search
• Two pointers

If asked for all permutations/subsets then

• Backtracking

If given a tree then

• DFS
• BFS

If given a graph then

• DFS
• BFS

If given a linked list then

• Two pointers

If recursion is banned then

• Stack

If must solve in-place then

• Swap corresponding values
• Store one or more different values in the same pointer

If asked for maximum/minimum subarray/subset/options then

• Dynamic programming

If asked for top/least K items then

• Heap
• QuickSelect

If asked for common strings then

• Map
• Trie

Else

• Map/Set for O(1) time & O(n) space
• Sort input for O(nlogn) time and O(1) space