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🧩 Coding Challenges

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Array

49 challenges · 18 easy · 24 med · 7 hard

1. Two Sum — Easy
2. Best Time to Buy and Sell Stock — Easy
3. Contains Duplicate — Easy
5. Maximum Subarray — Easy
18. Majority Element — Easy
109. Pascal's Triangle — Easy
112. Remove Duplicates from Sorted Array — Easy
113. Move Zeroes — Easy
116. Degree of an Array — Easy
201. Help the Dog Find Its Bone — Easy
205. Potion Shop — Can I Afford It? — Easy
206. Highest Score on the Leaderboard — Easy
209. Dragon's Gold Counter — Easy
212. Magic Sorting Hat — Assign Houses — Easy
213. Remove Duplicate Inventory Items — Easy
217. Flatten the Dungeon Map — Easy
223. Rotate the Carousel — Easy
224. Unique Quest Items — Easy
4. Product of Array Except Self — Medium
6. Merge Intervals — Medium
7. Rotate Array — Medium
8. Find Minimum in Rotated Sorted Array — Medium
9. 3Sum — Medium
10. 4Sum — Medium
11. Container With Most Water — Medium
17. Subarray Sum Equals K — Medium
19. Sort Colors — Medium
20. Next Permutation — Medium
101. Spiral Matrix — Medium
102. Set Matrix Zeroes — Medium
103. Rotate Image — Medium
104. Search a 2D Matrix — Medium
105. Maximum Product Subarray — Medium
106. Find All Duplicates in an Array — Medium
107. Jump Game II — Medium
108. Minimum Size Subarray Sum — Medium
110. Gas Station — Medium
115. Maximum Sum Circular Subarray — Medium
117. Longest Turbulent Subarray — Medium
118. Max Consecutive Ones III — Medium
119. Minimum Number of Arrows to Burst Balloons — Medium
120. Non-overlapping Intervals — Medium
12. Trapping Rain Water — Hard
13. First Missing Positive — Hard
14. Largest Rectangle in Histogram — Hard
15. Median of Two Sorted Arrays — Hard
16. Sliding Window Maximum — Hard
111. Candy — Hard
114. Count of Smaller Numbers After Self — Hard

Backtracking

17 challenges · 1 easy · 13 med · 3 hard

187. Binary Watch — Easy
80. Permutations — Medium
81. Subsets — Medium
82. Combination Sum — Medium
83. Palindrome Partitioning — Medium
84. Letter Combinations of a Phone Number — Medium
86. Word Search — Medium
180. Subsets II — Medium
181. Permutations II — Medium
182. Combination Sum II — Medium
183. Combination Sum III — Medium
184. Beautiful Arrangement — Medium
185. Restore IP Addresses — Medium
186. Letter Case Permutation — Medium
85. N-Queens — Hard
87. Sudoku Solver — Hard
254. Sudoku Solver — Backtracking — Hard

Binary Search

5 challenges · 2 easy · 3 med

93. Sqrt(x) — Easy
230. XP to Next Level — Binary Search — Easy
94. Search in Rotated Sorted Array — Medium
95. Find Peak Element — Medium
199. Search a 2D Matrix II — Medium

Binary Tree

25 challenges · 10 easy · 12 med · 3 hard

46. Maximum Depth of Binary Tree — Easy
47. Same Tree — Easy
48. Invert Binary Tree — Easy
56. Subtree of Another Tree — Easy
57. Symmetric Tree — Easy
146. Path Sum — Easy
153. Convert Sorted Array to BST — Easy
154. Balanced Binary Tree — Easy
155. Diameter of Binary Tree — Easy
156. Range Sum of BST — Easy
49. Binary Tree Level Order Traversal — Medium
50. Validate Binary Search Tree — Medium
51. Kth Smallest Element in a BST — Medium
52. Lowest Common Ancestor of a Binary Tree — Medium
55. Construct Binary Tree from Preorder and Inorder — Medium
147. Path Sum II — Medium
148. Sum Root to Leaf Numbers — Medium
149. Binary Tree Right Side View — Medium
150. Count Complete Tree Nodes — Medium
151. Flatten Binary Tree to Linked List — Medium
152. Binary Tree Zigzag Level Order Traversal — Medium
157. All Nodes Distance K in Binary Tree — Medium
53. Binary Tree Maximum Path Sum — Hard
54. Serialize and Deserialize Binary Tree — Hard
247. Serialize and Deserialize Binary Tree — Hard

Bit Manipulation

3 challenges · 3 easy

Breadth-First Search

1 challenge · 1 hard

242. Word Ladder — BFS Shortest Transformation — Hard

Depth-First Search

2 challenges · 1 med · 1 hard

238. Number of Islands — Medium
243. Reconstruct Itinerary — Eulerian Path — Hard

Design

5 challenges · 2 easy · 3 med

195. Design HashMap — Easy
196. Design HashSet — Easy
96. Implement Trie — Medium
244. LRU Cache — O(1) Get and Put — Medium
245. Implement a Trie (Prefix Tree) — Medium

Divide and Conquer

1 challenge · 1 hard

260. Count of Smaller Numbers After Self — Hard

Dynamic Programming

31 challenges · 1 easy · 18 med · 12 hard

58. Climbing Stairs — Easy
59. House Robber — Medium
60. Coin Change — Medium
61. Longest Increasing Subsequence — Medium
62. Unique Paths — Medium
63. Jump Game — Medium
64. Combination Sum IV — Medium
65. Decode Ways — Medium
69. Longest Common Subsequence — Medium
158. Triangle — Medium
159. Minimum Path Sum — Medium
161. Perfect Squares — Medium
162. Partition Equal Subset Sum — Medium
163. Target Sum — Medium
165. Palindromic Substrings — Medium
166. Ones and Zeroes — Medium
167. House Robber II — Medium
168. Maximal Square — Medium
169. Stone Game — Medium
66. Edit Distance — Hard
67. Best Time to Buy and Sell Stock IV — Hard
68. Burst Balloons — Hard
160. Dungeon Game — Hard
164. Interleaving String — Hard
231. Longest Increasing Subsequence — Hard
232. Edit Distance (Levenshtein Distance) — Hard
233. Burst Balloons — Hard
234. Palindrome Partitioning II — Minimum Cuts — Hard
235. Distinct Subsequences — Hard
237. Wildcard Pattern Matching — Hard
259. Maximum Profit with k Transactions — Hard

Graph

23 challenges · 1 easy · 17 med · 5 hard

174. Find the Town Judge — Easy
70. Number of Islands — Medium
71. Clone Graph — Medium
72. Course Schedule — Medium
73. Pacific Atlantic Water Flow — Medium
74. Graph Valid Tree — Medium
77. Network Delay Time — Medium
78. Cheapest Flights Within K Stops — Medium
170. Surrounded Regions — Medium
171. Number of Connected Components in an Undirected Graph — Medium
172. Redundant Connection — Medium
173. Is Graph Bipartite? — Medium
175. Evaluate Division — Medium
176. Minimum Height Trees — Medium
177. All Paths From Source to Target — Medium
178. Path With Minimum Effort — Medium
239. Course Schedule — Cycle Detection in Directed Graph — Medium
241. Network Delay Time — Dijkstra — Medium
75. Word Ladder — Hard
76. Alien Dictionary — Hard
79. Critical Connections in a Network — Hard
179. Swim in Rising Water — Hard
240. Alien Dictionary — Topological Sort — Hard

Greedy

2 challenges · 2 med

252. Jump Game II — Minimum Jumps — Medium
256. Task Scheduler — CPU Cooldown — Medium

Hash Table

3 challenges · 2 easy · 1 med

219. Word Count in a Story — Easy
222. Group Heroes by Class — Easy
257. Longest Consecutive Sequence — Medium

Heap

7 challenges · 5 med · 2 hard

99. Kth Largest Element in an Array — Medium
100. Top K Frequent Elements — Medium
198. Kth Smallest Element in a Sorted Matrix — Medium
253. Meeting Rooms II — Minimum Conference Rooms — Medium
255. K Closest Points to Origin — Medium
197. Find Median from Data Stream — Hard
246. Find Median from Data Stream — Hard

Linked List

20 challenges · 6 easy · 12 med · 2 hard

36. Reverse Linked List — Easy
37. Merge Two Sorted Lists — Easy
40. Linked List Cycle — Easy
136. Intersection of Two Linked Lists — Easy
137. Remove Duplicates from Sorted List — Easy
145. Palindrome Linked List — Easy
38. Add Two Numbers — Medium
39. Remove Nth Node From End of List — Medium
41. Linked List Cycle II — Medium
42. Reorder List — Medium
44. Copy List with Random Pointer — Medium
45. LRU Cache — Medium
138. Partition List — Medium
139. Rotate List — Medium
140. Swap Nodes in Pairs — Medium
141. Flatten a Multilevel Doubly Linked List — Medium
142. Sort List — Medium
144. Remove Duplicates from Sorted List II — Medium
43. Merge k Sorted Lists — Hard
143. Reverse Nodes in k-Group — Hard

Math

16 challenges · 10 easy · 6 med

90. Missing Number — Easy
91. Reverse Integer — Easy
190. Happy Number — Easy
192. Excel Sheet Column Number — Easy
202. Lemonade Stand Calculator — Easy
207. FizzBuzz Quest — Easy
211. Level Up! What's My New Stats? — Easy
220. Fibonacci Quest — First N Numbers — Easy
226. Mission Timer — Format Seconds — Easy
228. Treasure Chest Combinations — Easy
92. Pow(x, n) — Medium
189. Count Primes — Medium
191. Factorial Trailing Zeroes — Medium
1003. Cosine Similarity Between Two Vectors — Medium
1004. Precision, Recall, and F1 — Medium
1007. Top-K Retrieval by Cosine Similarity — Medium

Matrix

1 challenge · 1 easy

215. Castle Defence — Count the Guards — Easy

Sliding Window

3 challenges · 1 med · 2 hard

258. Find All Anagram Substrings — Medium
249. Sliding Window Maximum — Hard
250. Minimum Window Substring — Hard

Sorting

2 challenges · 1 easy · 1 med

216. Score Board Sorter — Easy
200. H-Index — Medium

Stack

7 challenges · 3 easy · 3 med · 1 hard

97. Min Stack — Easy
193. Implement Queue using Stacks — Easy
194. Implement Stack using Queues — Easy
98. Evaluate Reverse Polish Notation — Medium
248. Implement a Min Stack — Medium
1006. Evaluate a Postfix (RPN) Expression — Medium
236. Maximum Rectangle in Histogram — Hard

String

44 challenges · 20 easy · 16 med · 8 hard

21. Valid Anagram — Easy
22. Valid Palindrome — Easy
25. Palindrome Number — Easy
27. Longest Common Prefix — Easy
31. Valid Parentheses — Easy
121. Reverse String — Easy
124. Implement strStr() — Easy
132. Roman to Integer — Easy
134. Add Binary — Easy
135. Isomorphic Strings — Easy
203. Fix the Birthday Invitation — Easy
204. Count the Stars — Easy
208. Reverse the Hero's Name — Easy
210. Spell Check: Is It a Palindrome? — Easy
214. Translate to Pig Latin — Easy
218. Emoji Health Bar — Easy
221. Check the Magic Password — Easy
225. Title Case the Story — Easy
227. Anagram Detector — Easy
229. Snake Game — Find the Longest Run — Easy
23. Longest Substring Without Repeating Characters — Medium
24. Longest Palindromic Substring — Medium
26. Group Anagrams — Medium
28. String to Integer (atoi) — Medium
29. ZigZag Conversion — Medium
32. Generate Parentheses — Medium
34. Word Break — Medium
122. Reverse Words in a String — Medium
123. Count and Say — Medium
125. Longest Repeating Character Replacement — Medium
126. Decode String — Medium
127. Find All Anagrams in a String — Medium
128. Repeated DNA Sequences — Medium
131. Integer to Roman — Medium
133. Multiply Strings — Medium
1005. Tokenize an Arithmetic Expression — Medium
30. Minimum Window Substring — Hard
33. Longest Valid Parentheses — Hard
35. Regular Expression Matching — Hard
129. Text Justification — Hard
130. Wildcard Matching — Hard
1001. Constant-Time String Compare — Hard
1002. XOR Decryption: Single-Byte Key — Hard
1008. Recursive Text Splitter (RAG Chunker) — Hard

Two Pointers

1 challenge · 1 hard

251. Trapping Rain Water — Hard