##Leetcode Order by Frequency
| ID | QUESTION | DIFFICULTY | FREQENCY | DATA STRUCTURES | ALGORITHMS |
|---|---|---|---|---|---|
| 1 | Two Sum | 2 | 5 | array, set | sort, two pointers |
| 8 | String to Integer (atoi) | 2 | 5 | string | math |
| 15 | 3Sum | 3 | 5 | array | two pointers |
| 20 | Valid Parentheses | 2 | 5 | string | stack |
| 21 | Merge Two Sorted Lists | 2 | 5 | linked list | sort, two pointers, merge |
| 28 | Implement strStr() | 4 | 5 | string | two pointers, KMP, rolling hash |
| 50 | Pow(x, n) | 3 | 5 | binary search, math | |
| 56 | Merge Intervals | 4 | 5 | array, linked list, red-black tree | sort, merge |
| 57 | Insert Interval | 4 | 5 | array, linked list, red-black tree | sort, merge |
| 65 | Valid Number | 2 | 5 | string | math |
| 70 | Climbing Stairs | 2 | 5 | dp | |
| 73 | Set Matrix Zeroes | 3 | 5 | array | |
| 88 | Merge Sorted Array | 2 | 5 | array | two pointers, merge |
| 98 | Validate Binary Search Tree | 3 | 5 | tree | dfs |
| 125 | Valid Palindrome | 2 | 5 | string | two pointers |
| 127 | Word Ladder | 3 | 5 | graph | bfs, shortest path |
| 2 | Add Two Numbers | 3 | 4 | linked list | two pointers, math |
| 12 | Integer to Roman | 3 | 4 | math | |
| 13 | Roman to Integer | 2 | 4 | math | |
| 22 | Generate Parentheses | 3 | 4 | string | dfs |
| 23 | Merge k Sorted Lists | 3 | 4 | linked list, heap | sort, two pointers, merge |
| 24 | Swap Nodes in Pairs | 2 | 4 | linked list | |
| 27 | Remove Element | 1 | 4 | array | two pointers |
| 46 | Permutations | 3 | 4 | array | permutation |
| 49 | Anagrams | 3 | 4 | string, hashtable | |
| 67 | Add Binary | 2 | 4 | string | two pointers, math |
| 69 | Sqrt(x) | 4 | 4 | binary search | |
| 77 | Combinations | 3 | 4 | combination | |
| 78 | Subsets | 3 | 4 | array | recursion, combination |
| 79 | Word Search | 3 | 4 | array | dfs |
| 91 | Decode Ways | 3 | 4 | string | recursion, dp |
| 102 | Binary Tree Level Order Traversal | 3 | 4 | tree | bfs |
| 129 | Sum Root to Leaf Numbers | 2 | 4 | tree | dfs |
| 131 | Palindrome Partitioning | 3 | 4 | string | dfs |
| 4 | Median of Two Sorted Arrays | 5 | 3 | array | binary search |
| 7 | Reverse Integer | 2 | 3 | math | |
| 10 | Regular Expression Matching | 5 | 3 | string | recursion, dp |
| 17 | Letter Combinations of a Phone Number | 3 | 3 | string | dfs |
| 19 | Remove Nth Node From End of List | 2 | 3 | linked list | two pointers |
| 26 | Remove Duplicates from Sorted Array | 1 | 3 | array | two pointers |
| 29 | Divide Two Integers | 4 | 3 | binary search, math | |
| 33 | Search in Rotated Sorted Array | 4 | 3 | array | binary search |
| 34 | Search for a Range | 4 | 3 | array | binary search |
| 39 | Combination Sum | 3 | 3 | array | combination |
| 43 | Multiply Strings | 4 | 3 | string | two pointers |
| 44 | Wildcard Matching | 5 | 3 | string | recursion, dp, greedy |
| 51 | N-Queens | 4 | 3 | array | dfs |
| 52 | N-Queens II | 4 | 3 | array | dfs |
| 53 | Maximum Subarray | 3 | 3 | array | dp |
| 62 | Unique Paths | 2 | 3 | array | dp |
| 63 | Unique Paths II | 3 | 3 | array | dp |
| 64 | Minimum Path Sum | 3 | 3 | array | dp |
| 72 | Edit Distance | 4 | 3 | string | dp |
| 74 | Search a 2D Matrix | 3 | 3 | array | binary search |
| 81 | Search in Rotated Sorted Array II | 5 | 3 | array | binary search |
| 82 | Remove Duplicates from Sorted List II | 3 | 3 | linked list | recursion, two pointers |
| 83 | Remove Duplicates from Sorted List | 1 | 3 | linked list | |
| 86 | Partition List | 3 | 3 | linked list | two pointers |
| 93 | Restore IP Addresses | 3 | 3 | string | dfs |
| 94 | Binary Tree Inorder Traversal | 4 | 3 | tree, hashtable | recursion, morris, stack |
| 103 | Binary Tree Zigzag Level Order Traversal | 4 | 3 | queue, tree | bfs, stack |
| 105 | Construct Binary Tree from Preorder and Inorder Traversal | 3 | 3 | array, tree | dfs |
| 106 | Construct Binary Tree from Inorder and Postorder Traversal | 3 | 3 | array, tree | dfs |
| 108 | Convert Sorted Array to Binary Search Tree | 2 | 3 | tree | dfs |
| 109 | Convert Sorted List to Binary Search Tree | 4 | 3 | linked list | recursion, two pointers |
| 112 | Path Sum | 1 | 3 | tree | dfs |
| 114 | Flatten Binary Tree to Linked List | 3 | 3 | tree | recursion, stack |
| 116 | Populating Next Right Pointers in Each Node | 3 | 3 | tree | dfs |
| 128 | Longest Consecutive Sequence | 4 | 3 | array | |
| 130 | Surrounded Regions | 4 | 3 | array | bfs, dfs |
| 132 | Palindrome Partitioning II | 4 | 3 | string | dp |
| 3 | Longest Substring Without Repeating Characters | 3 | 2 | string, hashtable | two pointers |
| 5 | Longest Palindromic Substring | 4 | 2 | string | |
| 9 | Palindrome Number | 2 | 2 | math | |
| 11 | Container With Most Water | 3 | 2 | array | two pointers |
| 18 | 4Sum | 3 | 2 | array | |
| 25 | Reverse Nodes in k-Group | 4 | 2 | linked list | recursion, two pointers |
| 31 | Next Permutation | 5 | 2 | array | permutation |
| 35 | Search Insert Position | 2 | 2 | array | |
| 36 | Valid Sudoku | 2 | 2 | array | |
| 37 | Sudoku Solver | 4 | 2 | array | dfs |
| 38 | Count and Say | 2 | 2 | string | two pointers |
| 40 | Combination Sum II | 4 | 2 | array | combination |
| 41 | First Missing Positive | 5 | 2 | array | sort |
| 42 | Trapping Rain Water | 4 | 2 | array | two pointers, stack |
| 45 | Jump Game II | 4 | 2 | array | |
| 47 | Permutations II | 4 | 2 | array | permutation |
| 48 | Rotate Image | 4 | 2 | array | |
| 54 | Spiral Matrix | 4 | 2 | array | |
| 55 | Jump Game | 3 | 2 | array | |
| 59 | Spiral Matrix II | 3 | 2 | array | |
| 61 | Rotate List | 3 | 2 | linked list | two pointers |
| 66 | Plus One | 1 | 2 | array | math |
| 68 | Text Justification | 4 | 2 | string | |
| 75 | Sort Colors | 4 | 2 | array | sort, two pointers |
| 76 | Minimum Window Substring | 4 | 2 | string | two pointers |
| 80 | Remove Duplicates from Sorted Array II | 2 | 2 | array | two pointers |
| 84 | Largest Rectangle in Histogram | 5 | 2 | array | stack |
| 87 | Scramble String | 5 | 2 | string | recursion, dp |
| 89 | Gray Code | 4 | 2 | combination | |
| 90 | Subsets II | 4 | 2 | array | recursion, combination |
| 92 | Reverse Linked List II | 3 | 2 | linked list | two pointers |
| 97 | Interleaving String | 5 | 2 | string | recursion, dp |
| 99 | Recover Binary Search Tree | 4 | 2 | tree | dfs |
| 101 | Symmetric Tree | 1 | 2 | tree | dfs |
| 110 | Balanced Binary Tree | 1 | 2 | tree | dfs |
| 113 | Path Sum II | 2 | 2 | tree | dfs |
| 115 | Distinct Subsequences | 4 | 2 | string | dp |
| 117 | Populating Next Right Pointers in Each Node II | 4 | 2 | tree | dfs |
| 124 | Binary Tree Maximum Path Sum | 4 | 2 | tree | dfs |
| 6 | ZigZag Conversion | 3 | 1 | string | |
| 14 | Longest Common Prefix | 2 | 1 | string | |
| 16 | 3Sum Closest | 3 | 1 | array | two pointers |
| 30 | Substring with Concatenation of All Words | 3 | 1 | string | two pointers |
| 32 | Longest Valid Parentheses | 4 | 1 | string | dp |
| 58 | Length of Last Word | 1 | 1 | string | |
| 60 | Permutation Sequence | 5 | 1 | permutation, math | |
| 71 | Simplify Path | 3 | 1 | string | stack |
| 85 | Maximal Rectangle | 5 | 1 | array | dp, stack |
| 95 | Unique Binary Search Trees II | 4 | 1 | tree | dp, dfs |
| 96 | Unique Binary Search Trees | 3 | 1 | tree | dp |
| 100 | Same Tree | 1 | 1 | tree | dfs |
| 104 | Maximum Depth of Binary Tree | 1 | 1 | tree | dfs |
| 107 | Binary Tree Level Order Traversal II | 3 | 1 | tree | bfs |
| 111 | Minimum Depth of Binary Tree | 1 | 1 | tree | dfs |
| 118 | Pascal's Triangle | 2 | 1 | array | |
| 119 | Pascal's Triangle II | 2 | 1 | array | |
| 120 | Triangle | 3 | 1 | array | dp |
| 121 | Best Time to Buy and Sell Stock | 2 | 1 | array | dp |
| 122 | Best Time to Buy and Sell Stock II | 3 | 1 | array | greedy |
| 123 | Best Time to Buy and Sell Stock III | 4 | 1 | array | dp |
| 126 | Word Ladder II | 1 | 1 |