Problem Library

Master these coding challenges to ace your next technical interview. Handpicked problems with personalized spaced repetition.

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Title	Difficulty	Topics
1. Two Sum	Easy	ArrayHash Table
2. Add Two Numbers	Medium	Linked ListMathRecursion
3. Longest Substring Without Repeating Characters	Medium	Hash TableStringSliding Window
4. Median of Two Sorted Arrays	Hard	ArrayBinary SearchDivide and Conquer
5. Longest Palindromic Substring	Medium	Two PointersStringDynamic Programming
6. Zigzag Conversion	Medium	String
7. Reverse Integer	Medium	Math
8. String to Integer (atoi)	Medium	String
9. Palindrome Number	Easy	Math
10. Regular Expression Matching	Hard	StringDynamic ProgrammingRecursion
11. Container With Most Water	Medium	ArrayTwo PointersGreedy
12. Integer to Roman	Medium	Hash TableMathString
13. Roman to Integer	Easy	Hash TableMathString
14. Longest Common Prefix	Easy	ArrayStringTrie
15. 3Sum	Medium	ArrayTwo PointersSorting
16. 3Sum Closest	Medium	ArrayTwo PointersSorting
17. Letter Combinations of a Phone Number	Medium	Hash TableStringBacktracking
18. 4Sum	Medium	ArrayTwo PointersSorting
19. Remove Nth Node From End of List	Medium	Linked ListTwo Pointers
20. Valid Parentheses	Easy	StringStack
21. Merge Two Sorted Lists	Easy	Linked ListRecursion
22. Generate Parentheses	Medium	StringDynamic ProgrammingBacktracking
23. Merge k Sorted Lists	Hard	Linked ListDivide and ConquerHeap (Priority Queue)+1 more
24. Swap Nodes in Pairs	Medium	Linked ListRecursion
25. Reverse Nodes in k-Group	Hard	Linked ListRecursion
26. Remove Duplicates from Sorted Array	Easy	ArrayTwo Pointers
27. Remove Element	Easy	ArrayTwo Pointers
28. Find the Index of the First Occurrence in a String	Easy	Two PointersStringString Matching
29. Divide Two Integers	Medium	MathBit Manipulation
30. Substring with Concatenation of All Words	Hard	Hash TableStringSliding Window
31. Next Permutation	Medium	ArrayTwo Pointers
32. Longest Valid Parentheses	Hard	StringDynamic ProgrammingStack
33. Search in Rotated Sorted Array	Medium	ArrayBinary Search
34. Find First and Last Position of Element in Sorted Array	Medium	ArrayBinary Search
35. Search Insert Position	Easy	ArrayBinary Search
36. Valid Sudoku	Medium	ArrayHash TableMatrix
37. Sudoku Solver	Hard	ArrayHash TableBacktracking+1 more
38. Count and Say	Medium	String
39. Combination Sum	Medium	ArrayBacktracking
40. Combination Sum II	Medium	ArrayBacktracking
41. First Missing Positive	Hard	ArrayHash Table
42. Trapping Rain Water	Hard	ArrayTwo PointersDynamic Programming+2 more
43. Multiply Strings	Medium	MathStringSimulation
44. Wildcard Matching	Hard	StringDynamic ProgrammingGreedy+1 more
45. Jump Game II	Medium	ArrayDynamic ProgrammingGreedy
46. Permutations	Medium	ArrayBacktracking
47. Permutations II	Medium	ArrayBacktrackingSorting
48. Rotate Image	Medium	ArrayMathMatrix
49. Group Anagrams	Medium	ArrayHash TableString+1 more
50. Pow(x, n)	Medium	MathRecursion
51. N-Queens	Hard	ArrayBacktracking
52. N-Queens II	Hard	Backtracking
53. Maximum Subarray	Medium	ArrayDivide and ConquerDynamic Programming
54. Spiral Matrix	Medium	ArrayMatrixSimulation
55. Jump Game	Medium	ArrayDynamic ProgrammingGreedy
56. Merge Intervals	Medium	ArraySorting
57. Insert Interval	Medium	Array
58. Length of Last Word	Easy	String
59. Spiral Matrix II	Medium	ArrayMatrixSimulation
60. Permutation Sequence	Hard	MathRecursion
61. Rotate List	Medium	Linked ListTwo Pointers
62. Unique Paths	Medium	MathDynamic ProgrammingCombinatorics
63. Unique Paths II	Medium	ArrayDynamic ProgrammingMatrix
64. Minimum Path Sum	Medium	ArrayDynamic ProgrammingMatrix
65. Valid Number	Hard	String
66. Plus One	Easy	ArrayMath
67. Add Binary	Easy	MathStringBit Manipulation+1 more
68. Text Justification	Hard	ArrayStringSimulation
69. Sqrt(x)	Easy	MathBinary Search
70. Climbing Stairs	Easy	MathDynamic ProgrammingMemoization
71. Simplify Path	Medium	StringStack
72. Edit Distance	Medium	StringDynamic Programming
73. Set Matrix Zeroes	Medium	ArrayHash TableMatrix
74. Search a 2D Matrix	Medium	ArrayBinary SearchMatrix
75. Sort Colors	Medium	ArrayTwo PointersSorting
76. Minimum Window Substring	Hard	Hash TableStringSliding Window
77. Combinations	Medium	Backtracking
78. Subsets	Medium	ArrayBacktrackingBit Manipulation
79. Word Search	Medium	ArrayStringBacktracking+2 more
80. Remove Duplicates from Sorted Array II	Medium	ArrayTwo Pointers
81. Search in Rotated Sorted Array II	Medium	ArrayBinary Search
82. Remove Duplicates from Sorted List II	Medium	Linked ListTwo Pointers
83. Remove Duplicates from Sorted List	Easy	Linked List
84. Largest Rectangle in Histogram	Hard	ArrayStackMonotonic Stack
85. Maximal Rectangle	Hard	ArrayDynamic ProgrammingStack+2 more
86. Partition List	Medium	Linked ListTwo Pointers
87. Scramble String	Hard	StringDynamic Programming
88. Merge Sorted Array	Easy	ArrayTwo PointersSorting
89. Gray Code	Medium	MathBacktrackingBit Manipulation
90. Subsets II	Medium	ArrayBacktrackingBit Manipulation
91. Decode Ways	Medium	StringDynamic Programming
92. Reverse Linked List II	Medium	Linked List
93. Restore IP Addresses	Medium	StringBacktracking
94. Binary Tree Inorder Traversal	Easy	StackTreeDepth-First Search+1 more
95. Unique Binary Search Trees II	Medium	Dynamic ProgrammingBacktrackingTree+2 more
96. Unique Binary Search Trees	Medium	MathDynamic ProgrammingTree+2 more
97. Interleaving String	Medium	StringDynamic Programming
98. Validate Binary Search Tree	Medium	TreeDepth-First SearchBinary Search Tree+1 more
99. Recover Binary Search Tree	Medium	TreeDepth-First SearchBinary Search Tree+1 more
100. Same Tree	Easy	TreeDepth-First SearchBreadth-First Search+1 more