Data Structures

Arrays, strings, stacks, queues, hash maps, trees and graphs.

156 problems

Title	Tags	Acceptance	Difficulty
1. Two Sum	Array · Hash Map	—	Easy
2. Valid Parentheses	Stack · String	—	Easy
3. Next Greater Element	Stack · Monotonic Stack	—	Medium
4. Sliding Window Maximum	Deque · Sliding Window	—	Hard
5. Element Frequency	Hash Map · Counting	—	Easy
6. Shortest Path in a Graph	Graph · BFS	—	Medium
7. Number of Islands	Graph · DFS · Matrix	—	Medium
8. Merge Intervals	Sorting · Intervals	—	Medium
9. Top K Frequent Elements	Hash Map · Sorting · Heap	—	Medium
10. Valid Anagram	String · Hash Map · Sorting	—	Easy
11. First Unique Character	String · Hash Map	—	Easy
12. Move Zeroes	Array · Two Pointers	—	Easy
13. Rotate Array	Array	—	Easy
14. Product of Array Except Self	Array · Prefix Products	—	Medium
15. Set Operations	Set	—	Easy
16. Sum of Array	Array	—	Easy
17. Minimum and Maximum	Array	—	Easy
18. Reverse an Array	Array · Two Pointers	—	Easy
19. Second Largest Distinct Element	Array	—	Easy
20. Count Even and Odd	Array	—	Easy
21. Prefix Sums	Array · Prefix Sum	—	Easy
22. Range Sum Queries	Prefix Sum	—	Medium
23. Left Rotation	Array	—	Easy
24. Remove Duplicates from Sorted Array	Two Pointers	—	Easy
25. Merge Two Sorted Arrays	Two Pointers · Merge	—	Easy
26. Negatives Before Positives	Array · Partition	—	Easy
27. Leaders in an Array	Array · Suffix Max	—	Easy
28. Equilibrium Index	Prefix Sum	—	Medium
29. Max Consecutive Ones	Array	—	Easy
30. Missing and Repeating	Array · Math	—	Medium
31. Best Subarray with Bounds	Kadane	—	Medium
32. Product Modulo	Modular Arithmetic	—	Easy
33. Pair with Given Difference	Hash Set	—	Medium
34. Count Pairs with Sum	Hash Map · Counting	—	Medium
35. Subarrays with Sum K	Prefix Sum · Hash Map	—	Medium
36. Longest Subarray with Sum K	Prefix Sum · Hash Map	—	Medium
37. Maximum Sum Window	Sliding Window	—	Easy
38. Shortest Subarray with Sum ≥ S	Sliding Window · Two Pointers	—	Medium
39. Count Inversions	Merge Sort · Divide and Conquer	—	Hard
40. Elements More Than n/3 Times	Counting	—	Medium
41. Stock Span	Monotonic Stack	—	Medium
42. Sort 0s, 1s and 2s	Dutch National Flag	—	Easy
43. Next Permutation	Permutation	—	Medium
44. Alternate Positive and Negative	Array	—	Medium
45. Find a Peak Element	Binary Search	—	Medium
46. Intersection of Two Arrays	Set	—	Easy
47. Union of Two Arrays	Set	—	Easy
48. Sort by Frequency	Sorting · Hash Map	—	Medium
49. First Repeating Element	Hash Map	—	Easy
50. Zig-Zag Arrangement	Greedy	—	Medium
51. Minimum Platforms	Sorting · Sweep Line	—	Medium
52. Chocolate Distribution	Sorting · Sliding Window	—	Easy
53. Maximum Product of Two	Array	—	Easy
54. Distinct Elements in Every Window	Sliding Window · Hash Map	—	Medium
55. Contains Duplicate	Hash Set	—	Easy
56. Nearby Duplicate	Hash Map · Sliding Window	—	Easy
57. Longest Consecutive Sequence	Hash Set	—	Medium
58. Top K Frequent Words	Hash Map · Sorting	—	Medium
59. Valid Sudoku	Hash Set	—	Medium
60. Zero-Sum Subarray Exists?	Prefix Sum · Hash Set	—	Easy
61. Count Distinct Values	Hash Set	—	Easy
62. Four Arrays Zero Sum	Hash Map · Meet in the Middle	—	Medium
63. Pairs with Equal Digit Sum	Hash Map	—	Medium
64. Distinct Pairs with Sum	Hash Set	—	Medium
65. First Unique After Each Step	Queue · Hash Map	—	Medium
66. Stack Operations	Stack	—	Easy
67. Queue Operations	Queue	—	Easy
68. Min Stack	Stack · Design	—	Medium
69. Evaluate Postfix Expression	Stack	—	Medium
70. Infix to Postfix	Stack · Shunting Yard	—	Medium
71. Next Greater Element (Circular)	Monotonic Stack	—	Medium
72. Largest Rectangle in a Histogram	Monotonic Stack	—	Hard
73. Remove K Digits	Monotonic Stack · Greedy	—	Medium
74. Asteroid Collision	Stack · Simulation	—	Medium
75. Sliding Window Minimum	Deque	—	Medium
76. Decode String	Stack · Parsing	—	Medium
77. Simplify Unix Path	Stack	—	Medium
78. Minimum Brackets to Add	Counting	—	Medium
79. Longest Valid Parentheses	Stack	—	Hard
80. First Negative in Every Window	Queue · Sliding Window	—	Easy
81. Binary Numbers 1 to N	Queue	—	Easy
82. The Celebrity	Stack · Elimination	—	Medium
83. Shortest Path (Unweighted)	BFS	—	Easy
84. Connected Components	DFS · Union-Find	—	Easy
85. Cycle in an Undirected Graph	Union-Find · DFS	—	Medium
86. Cycle in a Directed Graph	Topological Sort	—	Medium
87. Topological Order	Topological Sort	—	Medium
88. Shortest Path (Weighted)	Dijkstra	—	Medium
89. Minimum Spanning Tree	Kruskal · Union-Find	—	Medium
90. Is the Graph Bipartite?	BFS · Colouring	—	Medium
91. Diameter of a Tree	BFS · Trees	—	Medium
92. Tree Height from Parent Array	Trees	—	Easy
93. Friend Circles	DFS · Adjacency Matrix	—	Easy
94. Rotting Oranges	Multi-source BFS	—	Medium
95. Count Regions of Same Colour	DFS · Flood Fill	—	Easy
96. Word Ladder	BFS	—	Hard
97. Lowest Common Ancestor	Trees · LCA	—	Hard
98. Paths in a DAG	Topological Sort · DP	—	Medium
99. Knight's Shortest Path	BFS	—	Medium
100. Transpose a Matrix	Matrix	—	Easy
101. Add Two Matrices	Matrix	—	Easy
102. Multiply Two Matrices	Matrix	—	Medium
103. Rotate Matrix 90° Clockwise	Matrix	—	Medium
104. Spiral Order	Simulation	—	Medium
105. Spiral Matrix Generator	Simulation	—	Medium
106. Diagonal Difference	Matrix	—	Easy
107. Row with Maximum Sum	Matrix	—	Easy
108. Set Matrix Zeroes	Matrix	—	Medium
109. Search a Sorted Matrix	Staircase Search	—	Medium
110. Sub-matrix Sum Queries	2-D Prefix Sum	—	Medium
111. Island Perimeter	Grid	—	Easy
112. Negatives in a Sorted Matrix	Staircase	—	Easy
113. Toeplitz Matrix	Matrix	—	Easy
114. Game of Life — One Step	Simulation	—	Medium
115. Row with Most Ones	Binary Search	—	Easy
116. Running Median	Heaps	—	Hard
117. LRU Cache	Design · Linked Hash Map	—	Medium
118. Reverse a Linked List	Linked List	—	Easy
119. Middle of a Linked List	Two Pointers	—	Easy
120. Remove N-th Node from End	Two Pointers	—	Medium
121. Merge Two Sorted Lists	Linked List · Merge	—	Easy
122. Palindrome Linked List	Two Pointers	—	Easy
123. Remove Duplicates from Sorted List	Linked List	—	Easy
124. Rotate List Right	Linked List	—	Medium
125. Add Two Numbers as Lists	Linked List · Math	—	Medium
126. Partition List	Linked List	—	Medium
127. Odd-Even Linked List	Linked List	—	Medium
128. Swap Nodes in Pairs	Linked List	—	Medium
129. Reverse Nodes in K-Groups	Linked List	—	Hard
130. Inorder Traversal	Binary Tree · DFS	—	Easy
131. Preorder Traversal	Binary Tree · DFS	—	Easy
132. Postorder Traversal	Binary Tree · DFS	—	Easy
133. Level Order by Lines	BFS	—	Easy
134. Zigzag Level Order	BFS	—	Medium
135. Maximum Depth of a Binary Tree	DFS	—	Easy
136. Minimum Depth	BFS	—	Easy
137. Count and Sum of Nodes	Traversal	—	Easy
138. Count Leaves	Traversal	—	Easy
139. Symmetric Tree	Recursion	—	Easy
140. Height-Balanced Tree	DFS	—	Easy
141. Diameter of a Binary Tree	DFS	—	Easy
142. Root-to-Leaf Path Sum	DFS	—	Easy
143. Validate a BST	BST · Inorder	—	Medium
144. K-th Smallest in a BST	BST · Inorder	—	Medium
145. LCA in a BST	BST	—	Easy
146. Build a BST by Insertion	BST	—	Easy
147. Range Sum of a BST	BST · DFS	—	Easy
148. Right Side View	BFS	—	Medium
149. Left Side View	BFS	—	Easy
150. Level Sums and Widest Level	BFS	—	Medium
151. Same Tree	Recursion	—	Easy
152. K Largest Elements	Heap	—	Easy
153. Merge K Sorted Lists	Heap · Merge	—	Hard
154. Connect Ropes at Minimum Cost	Heap · Greedy	—	Medium
155. K-th Largest in a Stream	Heap	—	Medium
156. Task Scheduler	Greedy · Counting	—	Medium

156 of 156 problems