Learning Workspace / Problem Library

Master the patterns that matter.

A structured interview roadmap with visual traces, implementation plans, and production-quality Python solutions.

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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
TimeO(n) SpaceO(n)
Easy
2
Valid Anagram Animated
TimeO(n) SpaceO(1)
Easy
3
Two Sum Animated
TimeO(n) SpaceO(n)
Easy
4
Group Anagrams Animated
TimeO(m·n·log n) SpaceO(m·n)
Medium
5
Top K Frequent Elements Animated
TimeO(n) SpaceO(n)
Medium
6
Encode and Decode Strings Animated
TimeO(n) SpaceO(n)
Medium
7
Product of Array Except Self Animated
TimeO(n) SpaceO(1)
Medium
8
Valid Sudoku Animated
TimeO(1) SpaceO(1)
Medium
9
Longest Consecutive Sequence Animated
TimeO(n) SpaceO(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
TimeO(n) SpaceO(1)
Easy
11
Two Sum II - Input Array Is Sorted Animated
TimeO(n) SpaceO(1)
Medium
12
3Sum Animated
TimeO(n²) SpaceO(n)
Medium
13
Container With Most Water Animated
TimeO(n) SpaceO(1)
Medium
14
Trapping Rain Water Animated
TimeO(n) SpaceO(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
TimeO(n) SpaceO(1)
Easy
16
Longest Substring Without Repeating Characters Animated
TimeO(n) SpaceO(n)
Medium
17
Longest Repeating Character Replacement Animated
TimeO(n) SpaceO(1)
Medium
18
Permutation in String Animated
TimeO(n) SpaceO(1)
Medium
19
Minimum Window Substring Animated
TimeO(n) SpaceO(n)
Hard
20
Sliding Window Maximum Animated
TimeO(n) SpaceO(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
TimeO(n) SpaceO(n)
Easy
22
Min Stack Animated
TimeO(1) SpaceO(n)
Medium
23
Evaluate Reverse Polish Notation Animated
TimeO(n) SpaceO(n)
Medium
24
Generate Parentheses Animated
TimeO(4^n/√n) SpaceO(n)
Medium
25
Daily Temperatures Animated
TimeO(n) SpaceO(n)
Medium
26
Car Fleet Animated
TimeO(n log n) SpaceO(n)
Medium
27
Largest Rectangle in Histogram Animated
TimeO(n) SpaceO(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
TimeO(n) SpaceO(1)
Easy
36
Merge Two Sorted Lists Animated
TimeO(n) SpaceO(1)
Easy
37
Reorder List Animated
TimeO(n) SpaceO(1)
Medium
38
Remove Nth Node From End of List Animated
TimeO(n) SpaceO(1)
Medium
39
Copy List with Random Pointer Animated
TimeO(n) SpaceO(n)
Medium
40
Add Two Numbers Animated
TimeO(n) SpaceO(n)
Medium
41
Linked List Cycle Animated
TimeO(n) SpaceO(1)
Easy
42
Find the Duplicate Number Animated
TimeO(n) SpaceO(1)
Medium
43
LRU Cache Animated
TimeO(1) SpaceO(n)
Medium
44
Merge k Sorted Lists Animated
TimeO(n log k) SpaceO(log k)
Hard
45
Reverse Nodes in k-Group Animated
TimeO(n) SpaceO(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
TimeO(n) SpaceO(n)
Easy
47
Maximum Depth of Binary Tree Animated
TimeO(n) SpaceO(n)
Easy
48
Diameter of Binary Tree Animated
TimeO(n) SpaceO(n)
Easy
49
Balanced Binary Tree Animated
TimeO(n) SpaceO(n)
Easy
50
Same Tree Animated
TimeO(n) SpaceO(n)
Easy
51
Subtree of Another Tree Animated
TimeO(m·n) SpaceO(m)
Easy
52
Lowest Common Ancestor of a BST Animated
TimeO(n) SpaceO(n)
Medium
53
Binary Tree Level Order Traversal Animated
TimeO(n) SpaceO(n)
Medium
54
Binary Tree Right Side View Animated
TimeO(n) SpaceO(n)
Medium
55
Count Good Nodes in Binary Tree Animated
TimeO(n) SpaceO(n)
Medium
56
Validate Binary Search Tree Animated
TimeO(n) SpaceO(n)
Medium
57
Kth Smallest Element in a BST Animated
TimeO(H+k) SpaceO(H)
Medium
58
Construct Binary Tree from Preorder and Inorder Traversal Animated
TimeO(n) SpaceO(n)
Medium
59
Binary Tree Maximum Path Sum Animated
TimeO(n) SpaceO(n)
Hard
60
Serialize and Deserialize Binary Tree Animated
TimeO(n) SpaceO(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
TimeO(n) SpaceO(n)
Medium
62
Design Add and Search Words Data Structure Animated
TimeO(n) SpaceO(n)
Medium
63
Word Search II Animated
TimeO(m·n·4^L) SpaceO(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
TimeO(n log k) SpaceO(k)
Easy
65
Last Stone Weight Animated
TimeO(n log n) SpaceO(n)
Easy
66
K Closest Points to Origin Animated
TimeO(n log k) SpaceO(k)
Medium
67
Kth Largest Element in an Array Animated
TimeO(n) SpaceO(1)
Medium
68
Task Scheduler Animated
TimeO(n log n) SpaceO(n)
Medium
69
Design Twitter Animated
TimeO(n log n) SpaceO(n)
Medium
70
Find Median from Data Stream Animated
TimeO(log n) SpaceO(n)
Hard

Backtracking

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

9 problems 0 easy 8 med 1 hard
71
Subsets Animated
TimeO(n·2^n) SpaceO(n)
Medium
72
Combination Sum Animated
TimeO(n^(t/m)) SpaceO(t/m)
Medium
73
Permutations Animated
TimeO(n!) SpaceO(n)
Medium
74
Subsets II Animated
TimeO(n·2^n) SpaceO(n)
Medium
75
Combination Sum II Animated
TimeO(2^n) SpaceO(n)
Medium
76
Word Search Animated
TimeO(4^n) SpaceO(n)
Medium
77
Palindrome Partitioning Animated
TimeO(n·2^n) SpaceO(n)
Medium
78
Letter Combinations of a Phone Number Animated
TimeO(4^n) SpaceO(n)
Medium
79
N-Queens Animated
TimeO(n!) SpaceO(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
TimeO(m·n) SpaceO(m·n)
Medium
81
Clone Graph Animated
TimeO(V+E) SpaceO(V)
Medium
82
Max Area of Island Animated
TimeO(m·n) SpaceO(m·n)
Medium
83
Pacific Atlantic Water Flow Animated
TimeO(m·n) SpaceO(m·n)
Medium
84
Surrounded Regions Animated
TimeO(m·n) SpaceO(m·n)
Medium
85
Rotting Oranges Animated
TimeO(m·n) SpaceO(m·n)
Medium
86
Walls and Gates Animated
TimeO(m·n) SpaceO(m·n)
Medium
87
Course Schedule Animated
TimeO(V+E) SpaceO(V+E)
Medium
88
Course Schedule II Animated
TimeO(V+E) SpaceO(V+E)
Medium
89
Number of Connected Components in an Undirected Graph Animated
TimeO(V+E) SpaceO(V+E)
Medium
90
Graph Valid Tree Animated
TimeO(V+E) SpaceO(V+E)
Medium
91
Word Ladder Animated
TimeO(n·m²) SpaceO(n·m²)
Hard
92
Redundant Connection Animated
TimeO(n·α(n)) SpaceO(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
TimeO(E log E) SpaceO(E)
Hard
94
Min Cost to Connect All Points Animated
TimeO(n² log n) SpaceO(n²)
Medium
95
Network Delay Time Animated
TimeO((V+E) log V) SpaceO(V+E)
Medium
96
Swim in Rising Water Animated
TimeO(n² log n) SpaceO(n²)
Hard
97
Alien Dictionary Animated
TimeO(C) SpaceO(1)
Hard
98
Cheapest Flights Within K Stops Animated
TimeO(K·E) SpaceO(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
TimeO(n) SpaceO(1)
Easy
100
Min Cost Climbing Stairs Animated
TimeO(n) SpaceO(1)
Easy
101
House Robber Animated
TimeO(n) SpaceO(1)
Medium
102
House Robber II Animated
TimeO(n) SpaceO(1)
Medium
103
Longest Palindromic Substring Animated
TimeO(n²) SpaceO(1)
Medium
104
Palindromic Substrings Animated
TimeO(n²) SpaceO(1)
Medium
105
Decode Ways Animated
TimeO(n) SpaceO(n)
Medium
106
Coin Change Animated
TimeO(n·m) SpaceO(n)
Medium
107
Maximum Product Subarray Animated
TimeO(n) SpaceO(1)
Medium
108
Word Break Animated
TimeO(n³) SpaceO(n)
Medium
109
Longest Increasing Subsequence Animated
TimeO(n log n) SpaceO(n)
Medium
110
Partition Equal Subset Sum Animated
TimeO(n·sum) SpaceO(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
TimeO(m·n) SpaceO(m·n)
Medium
112
Longest Common Subsequence Animated
TimeO(m·n) SpaceO(m·n)
Medium
113
Best Time to Buy and Sell Stock with Cooldown Animated
TimeO(n) SpaceO(1)
Medium
114
Coin Change II Animated
TimeO(n·m) SpaceO(n)
Medium
115
Target Sum Animated
TimeO(n·sum) SpaceO(n·sum)
Medium
116
Interleaving String Animated
TimeO(m·n) SpaceO(m·n)
Medium
117
Longest Increasing Path in a Matrix Animated
TimeO(m·n) SpaceO(m·n)
Hard
118
Distinct Subsequences Animated
TimeO(m·n) SpaceO(m·n)
Hard
119
Edit Distance Animated
TimeO(m·n) SpaceO(m·n)
Medium
120
Burst Balloons Animated
TimeO(n³) SpaceO(n²)
Hard
121
Regular Expression Matching Animated
TimeO(m·n) SpaceO(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
TimeO(n) SpaceO(1)
Medium
123
Jump Game Animated
TimeO(n) SpaceO(1)
Medium
124
Jump Game II Animated
TimeO(n) SpaceO(1)
Medium
125
Gas Station Animated
TimeO(n) SpaceO(1)
Medium
126
Hand of Straights Animated
TimeO(n log n) SpaceO(n)
Medium
127
Merge Triplets to Form Target Triplet Animated
TimeO(n) SpaceO(1)
Medium
128
Partition Labels Animated
TimeO(n) SpaceO(1)
Medium
129
Valid Parenthesis String Animated
TimeO(n) SpaceO(1)
Medium

Intervals

Overlap detection and merging. Meeting rooms, calendar problems.

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

Math & Geometry

Number theory, matrix operations, and geometric algorithms.

8 problems 2 easy 6 med 0 hard
136
Rotate Image Animated
TimeO(n²) SpaceO(1)
Medium
137
Spiral Matrix Animated
TimeO(n²) SpaceO(n²)
Medium
138
Set Matrix Zeroes Animated
TimeO(m·n) SpaceO(1)
Medium
139
Happy Number Animated
TimeO(log n) SpaceO(log n)
Easy
140
Plus One Animated
TimeO(n) SpaceO(1)
Easy
141
Pow(x, n) Animated
TimeO(log n) SpaceO(log n)
Medium
142
Multiply Strings Animated
TimeO(m·n) SpaceO(m+n)
Medium
143
Detect Squares Animated
TimeO(n) SpaceO(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
TimeO(n) SpaceO(1)
Easy
145
Number of 1 Bits Animated
TimeO(1) SpaceO(1)
Easy
146
Counting Bits Animated
TimeO(n) SpaceO(n)
Easy
147
Reverse Bits Animated
TimeO(1) SpaceO(1)
Easy
148
Missing Number Animated
TimeO(n) SpaceO(1)
Easy
149
Sum of Two Integers Animated
TimeO(1) SpaceO(1)
Medium
150
Reverse Integer Animated
TimeO(log n) SpaceO(1)
Medium