LeetCode 75 Study Plan – 75 Handpicked Problems Across 23 DSA Topics
🔗 Leetcode's Study Plan
This is a collection of my solutions to LeetCode 75 problems, organized by problem number.
Each solution includes:
- Intuitive approach
- Example walkthrough
- Well-structured code
- Time and space complexity analysis
If you're a beginner preparing for coding interviews or you're just brushing up on fundamentals, the LeetCode 75 plan:
- Covers all critical DSA patterns
- Progresses from easy > medium > hard
- Builds problem-solving confidence
- Saves time by removing decision fatigue (no need to pick problems yourself)
It’s interview-focused, topic-based, and battle-tested.
Each topic is documented in a dedicated section with:
- A table of problems
- Problem link
- Data Structures used
- Time and space complexity
- Direct link to my solution in this repo
Perfect for revision, practice, and interview prep.
- Arrays / Strings
- Two Pointers
- Sliding Window
- Prefix Sum
- Hash Map / Hash Set
- Stack
- Queue
- Linked List
- Binary Tree (DFS)
- Binary Tree (BFS)
- Binary Search Tree
- Graphs (DFS)
- Graphs (BFS)
- Heap / Priority Queue
- Binary Search
- Backtracking
- Dynamic Programming (1D)
- Dynamic Programming (Multi Multidimensional)
- Bit Manipulation
- Trie
- Intervals
- Monotonic Stack
| Sr. No. | Problem Name | Data Structures | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 1 | Merge Strings Alternately | String, Two Pointers | O(n + m) | O(n + m) | Code |
| 2 | Greatest Common Divisor of Strings | String, Math | O(n + m) | O(n + m) | Code |
| 3 | Kids With the Greatest Number of Candies | Array | O(n) | O(n) | Code |
| 4 | Can Place Flowers | Array, Greedy | O(n) | O(1) | Code |
| 5 | Reverse Vowels of a String | String, Two Pointers | O(n) | O(n) | Code |
| 6 | Reverse Words in a String | String, Two Pointers | O(n) | O(n) | Code |
| 7 | Product of Array Except Self | Array, Prefix Sum | O(n) | O(n) | Code |
| 8 | Increasing Triplet Subsequence | Array, Greedy | O(n) | O(1) | Code |
| 9 | String Compression | Array, Two Pointers | O(n) | O(1) | Code |
| Sr. No. | Problem Name | Data Structures | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 10 | Move Zeroes | Array, Two Pointers | O(n) | O(1) | Code |
| 11 | Is Sequence | String, Two Pointers, Dynamic Programming | O(n) | O(1) | Code |
| 12 | Container With Most Water | Array, Two Pointers, Greedy | O(n) | O(1) | Code |
| 13 | Max Number of K-Sum Pairs | Array, Hash Table, Two Pointers, Sorting | O(n) | O(n) | Code |
| Sr. No. | Problem Name | Data Structures | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 14 | Maximum Average Subarray I | Array, Sliding Window | O(n) | O(1) | Code |
| 15 | Maximum Number of Vowels in a Substring of Given Length | String, Sliding Window | O(n) | O(1) | Code |
| 16 | Max Consecutive Ones III | Array, Binary Search, Sliding Window, Prefix Sum | O(n) | O(1) | Code |
| 17 | Longest Subarray of 1's After Deleting One Element | Array,Sliding Window, Dynamic Programming | O(n) | O(1) | Code |
| Sr. No. | Problem Name | Data Structures | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 18 | Find the Highest Altitude | Array, Prefix Sum | O(n) | O(1) | Code |
| 19 | Find Pivot Index | Array, Prefix Sum | O(n) | O(1) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 20 | Find the Difference of Two Arrays | Hash Set, Array | O(n + m) | O(n + m) | Code |
| 21 | Unique Number of Occurrences | Hash Map, Hash Set, Array | O(n) | O(n) | Code |
| 22 | Determine if Two Strings Are Close | Hash Map, Sorting, String, Counting | O(n + k log n) | O(k) | Code |
| 23 | Equal Row and Column Pairs | Hash Map, Array, Matrix, Simulation | O(n²) | O(n²) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 24 | Removing Stars From a String | Stack, String, Simulation | O(n) | O(n) | Code |
| 25 | Asteroid Collision | Stack, Array, Simulation | O(n) | O(n) | Code |
| 26 | Decode String | Stack, String, Recursion | O(n*k) | O(n) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 27 | Number of Recent Calls | Queue, Data Stream | O(n) | O(n) | Code |
| 28 | Dota2 Senate | Queue, Greedy, String | O(n) | O(n) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 29 | Delete the Middle Node of a Linked List | Linked List, Two Pointers | O(n) | O(1) | Code |
| 30 | Odd Even Linked List | Linked List | O(n) | O(1) | Code |
| 31 | Reverse Linked List | Linked List | O(n) | O(1) | Code |
| 32 | Maximum Twin Sum of a Linked List | Linked List, Two Pointers, Stack | O(n) | O(1) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 33 | Maximum Depth of Binary Tree | Binary Tree, DFS | O(n) | O(h) | Code |
| 34 | Leaf-Similar Trees | Binary Tree, DFS | O(n + m) | O(h1 + h2 + l) | Code |
| 35 | Count Good Nodes in Binary Tree | Binary Tree, DFS | O(n) | O(h) | Code |
| 36 | Path Sum III | Binary Tree, DFS | O(n²) | O(h) | Code |
| 37 | Longest ZigZag Path in a Binary Tree | Binary Tree, DFS, DP | O(n) | O(h) | Code |
| 38 | Lowest Common Ancestor of a Binary Tree | Binary Tree, DFS | O(n) | O(h) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 39 | Binary Tree Right Side View | Binary Tree, BFS | O(n) | O(w) | Code |
| 40 | Maximum Level Sum of a Binary Tree | Binary Tree, BFS | O(n) | O(w) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 41 | Search in a Binary Search Tree | Binary Search Tree, DFS | O(h) | O(h) | Code |
| 42 | Delete Node in a BST | Binary Search Tree, DFS | O(h) | O(h) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 43 | Keys and Rooms | Graph, DFS | O(V + E) | O(v) | Code |
| 44 | Number of Provinces | Union Find, Graph, DFS | O(n^2 * α(n)) | O(n) | Code |
| 45 | Reorder Routes to Make All Paths Lead to the City Zero | Graph, DFS | O(n) | O(n) | Code |
| 46 | Evaluate Division | Graph, DFS, Array, String, Union Find, Shortest Path | O(E + Q * V) | O(V + E) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 47 | Nearest Exit from Entrance in Maze | Graph, BFS, Matrix, Queue | O(m × n) | O(m × n) | Code |
| 48 | Rotting Oranges | Graph, BFS Matrix, Queue | O(m × n) | O(m × n) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 49 | Kth Largest Element in an Array | Array, Heap (Priority Queue) | O(n + k log n) | O(k) | Code |
| 50 | Smallest Number in Infinite Set | Hash Table, Heap | O(log n) | O(n) | Code |
| 51 | Maximum Subsequence Score | Array, Greedy, Heap | O(n log n) | O(n) | Code |
| 52 | Total Cost to Hire K Workers | Array, Two Pointers, Heap | O(n log n) | O(n) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 53 | Guess Number Higher or Lower | Binary Search, Interactive | O(log n) | O(1) | Code |
| 54 | Successful Pairs of Spells and Potions | Array, Two Pointers, Binary Search, Sorting | O(n log m) | O(n + m) | Code |
| 55 | Find Peak Element | Array, Binary Search | O(log n) | O(1) | Code |
| 56 | Koko Eating Bananas | Array, Binary Search | O(n log m) | O(1) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 57 | Letter Combinations of a Phone Number | Hash Table, String, Backtracking | O(4^n · n) | O(n) | Code |
| 58 | Combination Sum III | Array, Backtracking | O(9 choose k) | O(k) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 59 | N-th Tribonacci Number | Math, DP, Memoization | O(n) | O(n) | Code |
| 60 | Min Cost Climbing Stairs | Array, DP | O(n) | O(1) | Code |
| 61 | House Robber | Array, DP | O(n) | O(1) | Code |
| 62 | Domino and Tromino Tiling | DP | O(n) | O(n) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 63 | Unique Paths | Math, DP, Combinatorics | O(m × n) | O(m × n) | Code |
| 64 | Longest Common Subsequence | String, DP | O(m × n) | O(m × n) | Code |
| 65 | Best Time to Buy and Sell Stock with Transaction Fee | Array, DP, Greedy | O(n) | O(n) | Code |
| 66 | Edit Distance | String, DP | O(m × n) | O(m × n) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 67 | Counting Bits | DP, Bit Manipulation | O(n) | O(n) | Code |
| 68 | Single Number | Array, Bit Manipulation | O(n) | O(1) | Code |
| 69 | Minimum Flips to Make a OR b Equal to c | Bit Manipulation | O(1) (fixed bits) | O(1) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 70 | Implement Trie (Prefix Tree) | Trie, Hash Table, String, Design | O(m) | O(m × n) | Code |
| 71 | Search Suggestions System | Trie, Heap, Array, String, Binary Search, Sorting | O(m log n) | O(n × m) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 72 | Non-overlapping Intervals | Array, DP, Greedy, Sorting | O(n log n) | O(n) | Code |
| 73 | Minimum Number of Arrows to Burst Balloons | Array, Greedy, Sorting | O(n log n) | O(n) | Code |
| Sr. No. | Problem Name | Data Structures Used | Time Complexity | Space Complexity | Solution Link |
|---|---|---|---|---|---|
| 74 | Daily Temperatures | Array, Stack, Monotonic Stack | O(n) | O(n) | Code |
| 75 | Online Stock Span | Stack, Design, Monotonic Stack, Data Stream | O(n) | O(n) | Code |
Check out my companion repository:
🔗 DSA Simplified – A complete walkthrough of all major DSA topics, with LeetCode, GFG, and custom examples.
This project is open-sourced under the MIT License. Feel free to use, fork, and share!
See LICENSE for more details.
Thanks to everyone who’s exploring this repo.
I hope it helps you crack interviews, ace coding rounds, and most importantly – become a better problem solver.
If you found this helpful, ⭐ star the repo and share it with your peers!
Happy coding!
— Rikam Palkar