1 
Two Sum 
Easy 
Array

Hash Table

2 
Add Two Numbers 
Medium 
Linked List 
Math

3 
Longest Substring Without Repeating Characters 
Medium 
Hash Table 
Two Pointers

String

Sliding Window

4 
Median of Two Sorted Arrays 
Hard 
Array

Binary Search

Divide and Conquer

5 
Longest Palindromic Substring 
Medium 
Dynamic Programming

String

6 
ZigZag Conversion 
Medium 
String

7 
Reverse Integer 
Easy 
Math

8 
String to Integer (atoi) 
Medium 
Math

String

9 
Palindrome Number 
Easy 
Math

10 
Regular Expression Matching 
Hard 
String

Dynamic Programming

Backtracking

11 
Container With Most Water 
Medium 
TBD

12 
Integer to Roman 
Medium 
Math

String

13 
Roman to Integer 
Easy 
Math

String

14 
Longest Common Prefix 
Easy 
String

15 
3Sum 
Medium 
Array

Two Pointers

16 
3Sum Closest 
Medium 
Array

Two Pointers

17 
Letter Combinations of a Phone Number 
Medium 
String

Backtracking 
Depthfirst search 
Recursion

18 
4Sum 
Medium 
Array

Hash Table 
Two Pointers

19 
Remove Nth Node From End of List 
Medium 
Linked List 
Two Pointers

20 
Valid Parentheses 
Easy 
String

Stack

21 
Merge Two Sorted Lists 
Easy 
Linked List 
Recursion

22 
Generate Parentheses 
Medium 
String

Backtracking

23 
Merge k Sorted Lists 
Hard 
Linked List 
Divide and Conquer 
Heap

24 
Swap Nodes in Pairs 
Medium 
Linked List 
Recursion

25 
Reverse Nodes in kGroup 
Hard 
Linked List

26 
Remove Duplicates from Sorted Array 
Easy 
Array

Two Pointers

27 
Remove Element 
Easy 
Array

Two Pointers

28 
Implement strStr() 
Easy 
Two Pointers

String

29 
Divide Two Integers 
Medium 
Math

Binary Search

30 
Substring with Concatenation of All Words 
Hard 
Hash Table 
Two Pointers

String

31 
Next Permutation 
Medium 
Array

32 
Longest Valid Parentheses 
Hard 
String

Dynamic Programming

33 
Search in Rotated Sorted Array 
Medium 
Array

Binary Search

34 
Find First and Last Position of Element in Sorted Array 
Medium 
Array

Binary Search

35 
Search Insert Position 
Easy 
Array

Binary Search

36 
Valid Sudoku 
Medium 
Hash Table

37 
Sudoku Solver 
Hard 
Hash Table 
Backtracking

38 
Count and Say 
Medium 
String

39 
Combination Sum 
Medium 
Array

Backtracking

40 
Combination Sum II 
Medium 
Array

Backtracking

41 
First Missing Positive 
Hard 
Array

42 
Trapping Rain Water 
Hard 
Array

Two Pointers

Stack

43 
Multiply Strings 
Medium 
Math

String

44 
Wildcard Matching 
Hard 
String

Dynamic Programming

Backtracking 
Greedy

45 
Jump Game II 
Hard 
Array

Greedy

46 
Permutations 
Medium 
Backtracking

47 
Permutations II 
Medium 
Backtracking

48 
Rotate Image 
Medium 
Array

53 
Maximum Subarray 
Easy 
Array

Divide and Conquer 
Dynamic Programming

54 
Spiral Matrix 
Medium 
Array

55 
Jump Game 
Medium 
Array

Greedy

56 
Merge Intervals 
Medium 
Array

Sort

57 
Insert Interval 
Medium 
Array

Sort

58 
Length of Last Word 
Easy 
String

59 
Spiral Matrix II 
Medium 
Array

62 
Unique Paths 
Medium 
Array

Dynamic Programming

63 
Unique Paths II 
Medium 
Array

Dynamic Programming

64 
Minimum Path Sum 
Medium 
Array

Dynamic Programming

66 
Plus One 
Easy 
Array

73 
Set Matrix Zeroes 
Medium 
Array

74 
Search a 2D Matrix 
Medium 
Array

Binary Search

75 
Sort Colors 
Medium 
Array

Two Pointers

Sort

78 
Subsets 
Medium 
Array

Backtracking 
Bit Manipulation

79 
Word Search 
Medium 
Array

Backtracking

80 
Remove Duplicates from Sorted Array II 
Medium 
Array

Two Pointers

81 
Search in Rotated Sorted Array II 
Medium 
Array

Binary Search

84 
Largest Rectangle in Histogram 
Hard 
Array

Stack

85 
Maximal Rectangle 
Hard 
Array

Hash Table 
Dynamic Programming

Stack

88 
Merge Sorted Array 
Easy 
Array

Two Pointers

90 
Subsets II 
Medium 
Array

Backtracking

92 
Reverse Linked List II 
Medium 
Linked List

94 
Binary Tree Inorder Traversal 
Medium 
Tree

Hash Table 
Stack

95 
Unique Binary Search Trees II 
Medium 
Dynamic Programming

Tree

96 
Unique Binary Search Trees 
Medium 
Dynamic Programming

Tree

98 
Validate Binary Search Tree 
Medium 
Tree

Depthfirst Search

99 
Recover Binary Search Tree 
Hard 
Tree

Depthfirst Search

100 
Same Tree 
Easy 
Tree

Depthfirst Search
