NeetCode 150

Coding Interview Problems.

All 150 problems organized by pattern. Filter by difficulty, search by name, or jump to a category.

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Arrays & Hashing

Fundamental data structures. Master hash maps, sets, and array manipulation patterns.

9 problems 3 easy 6 med 0 hard
1
Contains Duplicate Animated
O(n) O(n)
Easy 2
Valid Anagram Animated
O(n) O(1)
Easy 3
Two Sum Animated
O(n) O(n)
Easy 4
Group Anagrams Animated
O(m·n·log n) O(m·n)
Medium 5
Top K Frequent Elements Animated
O(n) O(n)
Medium 6
Encode and Decode Strings Animated
O(n) O(n)
Medium 7
Product of Array Except Self Animated
O(n) O(1)
Medium 8
Valid Sudoku Animated
O(1) O(1)
Medium 9
Longest Consecutive Sequence Animated
O(n) O(n)
Medium

Two Pointers

Left and right pointer technique for sorted arrays and palindromes.

5 problems 1 easy 3 med 1 hard
10
Valid Palindrome Animated
O(n) O(1)
Easy 11
Two Sum II - Input Array Is Sorted Animated
O(n) O(1)
Medium 12
3Sum Animated
O(n²) O(n)
Medium 13
Container With Most Water Animated
O(n) O(1)
Medium 14
Trapping Rain Water Animated
O(n) O(1)
Hard

Sliding Window

Dynamic window over sequences. Solve substring and subarray problems in O(n).

6 problems 1 easy 3 med 2 hard
15
Best Time to Buy and Sell Stock Animated
O(n) O(1)
Easy 16
Longest Substring Without Repeating Characters Animated
O(n) O(n)
Medium 17
Longest Repeating Character Replacement Animated
O(n) O(1)
Medium 18
Permutation in String Animated
O(n) O(1)
Medium 19
Minimum Window Substring Animated
O(n) O(n)
Hard 20
Sliding Window Maximum Animated
O(n) O(n)
Hard

Stack

LIFO structure. Solve parentheses, monotonic stack, and expression evaluation problems.

7 problems 1 easy 5 med 1 hard
21
Valid Parentheses Animated
O(n) O(n)
Easy 22
Min Stack Animated
O(1) O(n)
Medium 23
Evaluate Reverse Polish Notation Animated
O(n) O(n)
Medium 24
Generate Parentheses Animated
O(4^n/√n) O(n)
Medium 25
Daily Temperatures Animated
O(n) O(n)
Medium 26
Car Fleet Animated
O(n log n) O(n)
Medium 27
Largest Rectangle in Histogram Animated
O(n) O(n)
Hard

Linked List

Node chains with pointer manipulation. Reversals, cycle detection, and two-pointer tricks.

11 problems 3 easy 6 med 2 hard
35
Reverse Linked List Animated
O(n) O(1)
Easy 36
Merge Two Sorted Lists Animated
O(n) O(1)
Easy 37
Reorder List Animated
O(n) O(1)
Medium 38
Remove Nth Node From End of List Animated
O(n) O(1)
Medium 39
Copy List with Random Pointer Animated
O(n) O(n)
Medium 40
Add Two Numbers Animated
O(n) O(n)
Medium 41
Linked List Cycle Animated
O(n) O(1)
Easy 42
Find the Duplicate Number Animated
O(n) O(1)
Medium 43
LRU Cache Animated
O(1) O(n)
Medium 44
Merge k Sorted Lists Animated
O(n log k) O(log k)
Hard 45
Reverse Nodes in k-Group Animated
O(n) O(1)
Hard

Trees

Hierarchical structures. DFS, BFS, BST operations, and tree construction.

15 problems 6 easy 7 med 2 hard
46
Invert Binary Tree Animated
O(n) O(n)
Easy 47
Maximum Depth of Binary Tree Animated
O(n) O(n)
Easy 48
Diameter of Binary Tree Animated
O(n) O(n)
Easy 49
Balanced Binary Tree Animated
O(n) O(n)
Easy 50
Same Tree Animated
O(n) O(n)
Easy 51
Subtree of Another Tree Animated
O(m·n) O(m)
Easy 52
Lowest Common Ancestor of a BST Animated
O(n) O(n)
Medium 53
Binary Tree Level Order Traversal Animated
O(n) O(n)
Medium 54
Binary Tree Right Side View Animated
O(n) O(n)
Medium 55
Count Good Nodes in Binary Tree Animated
O(n) O(n)
Medium 56
Validate Binary Search Tree Animated
O(n) O(n)
Medium 57
Kth Smallest Element in a BST Animated
O(H+k) O(H)
Medium 58
Construct Binary Tree from Preorder and Inorder Traversal Animated
O(n) O(n)
Medium 59
Binary Tree Maximum Path Sum Animated
O(n) O(n)
Hard 60
Serialize and Deserialize Binary Tree Animated
O(n) O(n)
Hard

Tries

Prefix trees for word lookup and autocomplete. Build and search word dictionaries.

3 problems 0 easy 2 med 1 hard
61
Implement Trie (Prefix Tree) Animated
O(n) O(n)
Medium 62
Design Add and Search Words Data Structure Animated
O(n) O(n)
Medium 63
Word Search II Animated
O(m·n·4^L) O(n·L)
Hard

Heap / Priority Queue

Efficient max/min retrieval. Solve k-th element, median stream, and scheduling problems.

7 problems 2 easy 4 med 1 hard
64
Kth Largest Element in a Stream Animated
O(n log k) O(k)
Easy 65
Last Stone Weight Animated
O(n log n) O(n)
Easy 66
K Closest Points to Origin Animated
O(n log k) O(k)
Medium 67
Kth Largest Element in an Array Animated
O(n) O(1)
Medium 68
Task Scheduler Animated
O(n log n) O(n)
Medium 69
Design Twitter Animated
O(n log n) O(n)
Medium 70
Find Median from Data Stream Animated
O(log n) O(n)
Hard

Backtracking

Explore all possibilities with pruning. Subsets, permutations, combinations.

9 problems 0 easy 8 med 1 hard
71
Subsets Animated
O(n·2^n) O(n)
Medium 72
Combination Sum Animated
O(n^(t/m)) O(t/m)
Medium 73
Permutations Animated
O(n!) O(n)
Medium 74
Subsets II Animated
O(n·2^n) O(n)
Medium 75
Combination Sum II Animated
O(2^n) O(n)
Medium 76
Word Search Animated
O(4^n) O(n)
Medium 77
Palindrome Partitioning Animated
O(n·2^n) O(n)
Medium 78
Letter Combinations of a Phone Number Animated
O(4^n) O(n)
Medium 79
N-Queens Animated
O(n!) O(n)
Hard

Graphs

Vertices and edges. DFS, BFS, topological sort, and union-find algorithms.

13 problems 0 easy 12 med 1 hard
80
Number of Islands Animated
O(m·n) O(m·n)
Medium 81
Clone Graph Animated
O(V+E) O(V)
Medium 82
Max Area of Island Animated
O(m·n) O(m·n)
Medium 83
Pacific Atlantic Water Flow Animated
O(m·n) O(m·n)
Medium 84
Surrounded Regions Animated
O(m·n) O(m·n)
Medium 85
Rotting Oranges Animated
O(m·n) O(m·n)
Medium 86
Walls and Gates Animated
O(m·n) O(m·n)
Medium 87
Course Schedule Animated
O(V+E) O(V+E)
Medium 88
Course Schedule II Animated
O(V+E) O(V+E)
Medium 89
Number of Connected Components in an Undirected Graph Animated
O(V+E) O(V+E)
Medium 90
Graph Valid Tree Animated
O(V+E) O(V+E)
Medium 91
Word Ladder Animated
O(n·m²) O(n·m²)
Hard 92
Redundant Connection Animated
O(n·α(n)) O(n)
Medium

Advanced Graphs

Dijkstra, Bellman-Ford, Kruskal/Prim MST, and Eulerian paths.

6 problems 0 easy 3 med 3 hard
93
Reconstruct Itinerary Animated
O(E log E) O(E)
Hard 94
Min Cost to Connect All Points Animated
O(n² log n) O(n²)
Medium 95
Network Delay Time Animated
O((V+E) log V) O(V+E)
Medium 96
Swim in Rising Water Animated
O(n² log n) O(n²)
Hard 97
Alien Dictionary Animated
O(C) O(1)
Hard 98
Cheapest Flights Within K Stops Animated
O(K·E) O(V)
Medium

1D Dynamic Programming

Memoize overlapping subproblems in one dimension. Fibonacci, knapsack, LCS patterns.

12 problems 2 easy 10 med 0 hard
99
Climbing Stairs Animated
O(n) O(1)
Easy 100
Min Cost Climbing Stairs Animated
O(n) O(1)
Easy 101
House Robber Animated
O(n) O(1)
Medium 102
House Robber II Animated
O(n) O(1)
Medium 103
Longest Palindromic Substring Animated
O(n²) O(1)
Medium 104
Palindromic Substrings Animated
O(n²) O(1)
Medium 105
Decode Ways Animated
O(n) O(n)
Medium 106
Coin Change Animated
O(n·m) O(n)
Medium 107
Maximum Product Subarray Animated
O(n) O(1)
Medium 108
Word Break Animated
O(n³) O(n)
Medium 109
Longest Increasing Subsequence Animated
O(n log n) O(n)
Medium 110
Partition Equal Subset Sum Animated
O(n·sum) O(sum)
Medium

2D Dynamic Programming

Grid-based DP. Edit distance, LCS, unique paths and string matching problems.

11 problems 0 easy 7 med 4 hard
111
Unique Paths Animated
O(m·n) O(m·n)
Medium 112
Longest Common Subsequence Animated
O(m·n) O(m·n)
Medium 113
Best Time to Buy and Sell Stock with Cooldown Animated
O(n) O(1)
Medium 114
Coin Change II Animated
O(n·m) O(n)
Medium 115
Target Sum Animated
O(n·sum) O(n·sum)
Medium 116
Interleaving String Animated
O(m·n) O(m·n)
Medium 117
Longest Increasing Path in a Matrix Animated
O(m·n) O(m·n)
Hard 118
Distinct Subsequences Animated
O(m·n) O(m·n)
Hard 119
Edit Distance Animated
O(m·n) O(m·n)
Medium 120
Burst Balloons Animated
O(n³) O(n²)
Hard 121
Regular Expression Matching Animated
O(m·n) O(m·n)
Hard

Greedy

Local optimum leads to global optimum. Interval scheduling, activity selection.

8 problems 0 easy 8 med 0 hard
122
Maximum Subarray Animated
O(n) O(1)
Medium 123
Jump Game Animated
O(n) O(1)
Medium 124
Jump Game II Animated
O(n) O(1)
Medium 125
Gas Station Animated
O(n) O(1)
Medium 126
Hand of Straights Animated
O(n log n) O(n)
Medium 127
Merge Triplets to Form Target Triplet Animated
O(n) O(1)
Medium 128
Partition Labels Animated
O(n) O(1)
Medium 129
Valid Parenthesis String Animated
O(n) O(1)
Medium

Intervals

Overlap detection and merging. Meeting rooms, calendar problems.

6 problems 1 easy 4 med 1 hard
130
Insert Interval Animated
O(n) O(n)
Medium 131
Merge Intervals Animated
O(n log n) O(n)
Medium 132
Non-overlapping Intervals Animated
O(n log n) O(1)
Medium 133
Meeting Rooms Animated
O(n log n) O(1)
Easy 134
Meeting Rooms II Animated
O(n log n) O(n)
Medium 135
Minimum Interval to Include Each Query Animated
O((n+q) log n) O(n+q)
Hard

Math & Geometry

Number theory, matrix operations, and geometric algorithms.

8 problems 2 easy 6 med 0 hard
136
Rotate Image Animated
O(n²) O(1)
Medium 137
Spiral Matrix Animated
O(n²) O(n²)
Medium 138
Set Matrix Zeroes Animated
O(m·n) O(1)
Medium 139
Happy Number Animated
O(log n) O(log n)
Easy 140
Plus One Animated
O(n) O(1)
Easy 141
Pow(x, n) Animated
O(log n) O(log n)
Medium 142
Multiply Strings Animated
O(m·n) O(m+n)
Medium 143
Detect Squares Animated
O(n) O(n)
Medium
010

Bit Manipulation

XOR, bit shifts, and masks. Solve space-optimized problems with bitwise logic.

7 problems 5 easy 2 med 0 hard
144
Single Number Animated
O(n) O(1)
Easy 145
Number of 1 Bits Animated
O(1) O(1)
Easy 146
Counting Bits Animated
O(n) O(n)
Easy 147
Reverse Bits Animated
O(1) O(1)
Easy 148
Missing Number Animated
O(n) O(1)
Easy 149
Sum of Two Integers Animated
O(1) O(1)
Medium 150
Reverse Integer Animated
O(log n) O(1)
Medium