No. Problem Stumped Algorithms time complexity 001 Two Sum Easy***** HashMap O(N) 002 Add Two Numbers Medium**** LinkedList O(max(M,N)) 008 String to Integer (atoi) Easy***** - O(N) 012 Integer to Roman Medium**** divide O(1) 013 Roman to Integer Easy**** - O(N) 015 3Sum Medium***** traverse O(N2) 016 3Sum Closest Medium***** traverse O(NlogN) 020 Valid Parentheses Easy***** stack O(N) 021 Merge Two Sorted Lists Easy***** recursion O(N) 022 Generate Parentheses Medium**** DFS Stack - 023 Merge k Sorted Lists Hard**** divide - 024 Swap Nodes in Pairs Medium**** recursive O(N) 027 Remove Element Easy**** - O(N) 028 Implement strStr() Easy***** - O(M*N) 039 Combination Sum Medium**** backtrack - 040 Combination Sum II Medium**** backtrack - 046 Permutations Medium**** backtrack DFS - 047 Permutations II Medium**** backtrack - 049 Anagrams Medium**** HashMap - 056 Merge Intervals Hard***** - O(N) 057 Insert Interval Hard***** - O(N) 065 Valid Number Hard***** ifs - 067 Add Binary Easy**** - O(N) 069 Sqrt(x) Easy**** dichotomy - 070 Climbing Stairs Easy***** Dynamic Programming - 077 Combinations Medium**** backtrack - 078 Subsets Medium**** backtrack - 079 Word Search Medium**** backtrack DFS - 088 Merge Sorted Array Easy***** - O(N) 090 Subsets II Medium**** backtrack - 091 Decode Ways Medium**** DP - 098 Validate Binary Search Tree Medium***** DFS - 102 Binary Tree Level Order Traversal Easy**** BFS Queue - 125 Valid Palindrome Easy***** recursion - 127 Word Ladder Medium***** Breadth-First Search - 129 Sum Root to Leaf Numbers Medium**** Depth-First Search - 131 Palindrome Partitioning Medium**** backtrack -