# LeetCode Statistics

### LeetCode Statistics

ID Question Diff Freq Data Structure Algorithms

1 Two Sum 2 5 array sort

set Two Pointers

Math

3 Longest Substring Without Repeating Characters 3 2 string Two Pointers

hashtable

4 Median of Two Sorted Arrays 5 3 array Binary Search

5 Longest Palindromic Substring 4 2 string

6 ZigZag Conversion 3 1 string

7 Reverse Integer 2 3   Math

8 String to Integer (atoi) 2 5 string Math

9 Palindrome Number 2 2   Math

10 Regular Expression Matching 5 3 string Recursion

DP

11 Container With Most Water 3 2 array Two Pointers

12 Integer to Roman 3 4   Math

13 Roman to Integer 2 4   Math

14 Longest Common Prefix 2 1 string

15 3Sum 3 5 array Two Pointers

16 3Sum Closest 3 1 array Two Pointers

17 Letter Combinations of a Phone Number 3 3 string DFS

18 4Sum 3 2 array

19 Remove Nth Node From End of List 2 3 linked list Two Pointers

20 Valid Parentheses 2 5 string Stack

21 Merge Two Sorted Lists 2 5 linked list sort

Two Pointers

merge

22 Generate Parentheses 3 4 string DFS

23 Merge k Sorted Lists 3 4 linked list sort

heap Two Pointers

merge

24 Swap Nodes in Pairs 2 4 linked list

25 Reverse Nodes in k-Group 4 2 linked list Recursion

Two Pointers

26 Remove Duplicates from Sorted Array 1 3 array Two Pointers

27 Remove Element 1 4 array Two Pointers

28 Implement strStr() 4 5 string Two Pointers

KMP

rolling hash

29 Divide Two Integers 4 3   Binary Search

Math

30 Substring with Concatenation of All Words 3 1 string Two Pointers

31 Next Permutation 5 2 array permutation

32 Longest Valid Parentheses 4 1 string DP

33 Search in Rotated Sorted Array 4 3 array Binary Search

34 Search for a Range 4 3 array Binary Search

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

39 Combination Sum 3 3 array combination

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

43 Multiply Strings 4 3 string Two Pointers

Math

44 Wildcard Matching 5 3 string Recursion

DP

greedy

45 Jump Game II 4 2 array

46 Permutations 3 4 array permutation

47 Permutations II 4 2 array permutation

48 Rotate Image 4 2 array

49 Anagrams 3 4 string

hashtable

50 Pow(x, n) 3 5   Binary Search

Math

51 N-Queens 4 3 array DFS

52 N-Queens II 4 3 array DFS

53 Maximum Subarray 3 3 array DP

54 Spiral Matrix 4 2 array

55 Jump Game 3 2 array

56 Merge Intervals 4 5 array sort

red-black tree

57 Insert Interval 4 5 array sort

red-black tree

58 Length of Last Word 1 1 string

59 Spiral Matrix II 3 2 array

60 Permutation Sequence 5 1   permutation

Math

61 Rotate List 3 2 linked list Two Pointers

62 Unique Paths 2 3 array DP

63 Unique Paths II 3 3 array DP

64 Minimum Path Sum 3 3 array DP

65 Valid Number 2 5 string Math

66 Plus One 1 2 array Math

67 Add Binary 2 4 string Two Pointers

Math

68 Text Justification 4 2 string

69 Sqrt(x) 4 4   Binary Search

70 Climbing Stairs 2 5   DP

71 Simplify Path 3 1 string Stack

72 Edit Distance 4 3 string DP

73 Set Matrix Zeroes 3 5 array

74 Search a 2D Matrix 3 3 array Binary Search

75 Sort Colors 4 2 array sort

Two Pointers

76 Minimum Window Substring 4 2 string Two Pointers

77 Combinations 3 4   combination

78 Subsets 3 4 array Recursion

combination

79 Word Search 3 4 array DFS

80 Remove Duplicates from Sorted Array II 2 2 array Two Pointers

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

84 Largest Rectangle in Histogram 5 2 array Stack

85 Maximal Rectangle 5 1 array DP

Stack

86 Partition List 3 3 linked list Two Pointers

87 Scramble String 5 2 string Recursion

DP

88 Merge Sorted Array 2 5 array Two Pointers

merge

89 Gray Code 4 2   combination

90 Subsets II 4 2 array Recursion

combination

91 Decode Ways 3 4 string Recursion

DP

93 Restore IP Addresses 3 3 string DFS

94 Binary Tree Inorder Traversal 4 3 tree Recursion

hashtable morris

Stack

95 Unique Binary Search Trees II 4 1 tree DP

DFS

96 Unique Binary Search Trees 3 1 tree DP

97 Interleaving String 5 2 string Recursion

DP

98 Validate Binary Search Tree 3 5 tree DFS

99 Recover Binary Search Tree 4 2 tree DFS

100 Same Tree 1 1 tree DFS

101 Symmetric Tree 1 2 tree DFS

102 Binary Tree Level Order Traversal 3 4 tree BFS

103 Binary Tree Zigzag Level Order Traversal 4 3 queue BFS

tree Stack

104 Maximum Depth of Binary Tree 1 1 tree DFS

105 Construct Binary Tree from Preorder and Inorder Tr 3 3 array DFS

tree

106 Construct Binary Tree from Inorder and Postorder T 3 3 array DFS

tree

107 Binary Tree Level Order Traversal II 3 1 tree BFS

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

110 Balanced Binary Tree 1 2 tree DFS

111 Minimum Depth of Binary Tree 1 1 tree DFS

112 Path Sum 1 3 tree DFS

113 Path Sum II 2 2 tree DFS

114 Flatten Binary Tree to Linked List 3 3 tree Recursion

Stack

115 Distinct Subsequences 4 2 string DP

116 Populating Next Right Pointers in Each Node 3 3 tree DFS

117 Populating Next Right Pointers in Each Node II 4 2 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

124 Binary Tree Maximum Path Sum 4 2 tree DFS

125 Valid Palindrome 2 5 string Two Pointers

126 Word Ladder II 1 1

127 Word Ladder 3 5 graph BFS

shortest path

128 Longest Consecutive Sequence 4 3 array

129 Sum Root to Leaf Numbers 2 4 tree DFS

130 Surrounded Regions 4 3 array BFS

DFS

131 Palindrome Partitioning 3 4 string DFS

132 Palindrome Partitioning II 4 3 string DP

### Dynamic Programming

Edit Distance Maximum Subarray Minimum Path Sum Unique Paths Unique Paths II Longest Palindromic Substring Interleaving String Triangle Distinct Subsequences Decode Ways Palindrome Partitioning II Maximal Rectangle

### Recursion

N-Queens N-Queens II Balanced Binary Tree Binary Tree Inorder Traversal Binary Tree Maximum Path Sum Convert Sorted Array to Binary Search Tree Convert Sorted List to Binary Search Tree Flatten Binary Tree to Linked List Maximum Depth of Binary Tree Minimum Depth of Binary Tree Path Sum Permutations Permutations II Populating Next Right Pointers in Each Node Pow(x, n) Same Tree Subsets Sum Root to Leaf Numbers Swap Nodes in Pairs Symmetric Tree Valid Palindrome Validate Binary Search Tree Restore IP Addresses Combinations Interleaving String (dp is the best) Combination Sum II Letter Combinations of a Phone Numbers Word Search Construct Binary Tree from Inorder and Postorder Traversal Construct Binary Tree from Preorder and Inorder Traversal Generate Parentheses Surrounded Regions (runtime error) Palindrome Partitioning Combination Sum Sudoku Solver Unique Binary Search Trees II

### Binary Search

Search Insert Position Search a 2D Matrix Search for a Range Search in Rotated Sorted Array Sqrt(x)

### Sequence

Container With Most Water Count and Say First Missing Positive Implement strStr() Jump Game Jump Game II Length of Last Word Longest Common Prefix Longest Substring Without Repeating Characters Merge Sorted Array Palindrome Number Plus One Remove Duplicates from Sorted Array Remove Duplicates from Sorted Array II Remove Element Reverse Integer Search in Rotated Sorted Array II Sort Colors Two Sum 3Sum 3Sum Closest 4Sum Add Binary Longest Palindromic Substring Next Permutation Longest Valid Parentheses Climbing Stairs Permutation Sequence Simplify Path String to Integer (atoi) Minimum Window Substring Longest Consecutive Sequence Trapping Rain Water Valid Number

Add Two Numbers Convert Sorted List to Binary Search Tree Merge Two Sorted Lists Partition List Remove Duplicates from Sorted List Remove Duplicates from Sorted List II Remove Nth Node From End of List Reverse Linked List II Reverse Nodes in k-Group Rotate List Swap Nodes in Pairs

### Stack

Binary Tree Inorder Traversal Binary Tree Level Order Traversal II Valid Parentheses

### Queue

Binary Tree Level Order Traversal Binary Tree Level Order Traversal II Populating Next Right Pointers in Each Node II Symmetric Tree Surrounded Regions Word Ladder

### Tree

Balanced Binary Tree Binary Tree Inorder Traversal Binary Tree Level Order Traversal Binary Tree Level Order Traversal II Binary Tree Maximum Path Sum Convert Sorted Array to Binary Search Tree Convert Sorted List to Binary Search Tree Flatten Binary Tree to Linked List Maximum Depth of Binary Tree Minimum Depth of Binary Tree Path Sum Same Tree Sum Root to Leaf Numbers Symmetric Tree Validate Binary Search Tree