← Back to DSA overview

Hand-written tutorials, ordered as a recommended learning path.

1. 81 Queue via Two Stacks Implement a queue using two stacks with amortized O(1) operations. Covers costly enqueue vs costly dequeue approaches with Python code. 5 min read →
2. 82 Two Pointer Advanced Master opposite-direction, same-direction, and fast-slow two pointer patterns. Includes templates, container with most water, cycle detection, and sort colors. 12 min read →
3. 83 Stack via Two Queues Implement a stack using two queues with costly push and costly pop approaches. Complete Python solutions with complexity analysis. 4 min read →
4. 84 Min Remove Parentheses Solve LeetCode 1249 Minimum Remove to Make Valid Parentheses using a stack. Two-pass and one-pass approaches with Python code and traces. 5 min read →
5. 85 Basic Calculator I/II/III Solve Basic Calculator problems LeetCode 224, 227, and 772. Master stack-based expression evaluation with +, -, *, /, and parentheses in Python. 6 min read →
6. 86 Reverse Polish Notation Solve LeetCode 150 Evaluate Reverse Polish Notation using a stack. Python implementation with division gotcha, traces, and complexity analysis. 5 min read →
7. 87 Deque Design Patterns Master deque design patterns including sliding window maximum, palindrome checking, work stealing, and BFS/DFS hybrid. Python implementations. 7 min read →
8. 88 Sliding Window Advanced Master fixed-size and variable-size sliding window techniques. Covers minimum window substring, longest substring with K distinct chars, and string permutation problems. 11 min read →
9. 89 Simplify Unix Path Solve Simplify Path LeetCode 71 with a stack. Handle ., .., multiple slashes, and edge cases. Python solution with step-by-step trace. 4 min read →
10. 90 Remove Duplicate Letters Solve LeetCode 316 Remove Duplicate Letters using monotonic stack with frequency and visited tracking. Smallest lexicographic subsequence in Python. 5 min read →
11. 91 Car Fleet Solve LeetCode 853 Car Fleet using a stack. Sort by position, compare arrival times, and count fleets. Python solution with visual trace. 5 min read →
12. 92 Priority Queue Patterns Master priority queue patterns for coding interviews. Top-K elements, merge K sorted lists, running median, and Dijkstra's algorithm in Python. 7 min read →
13. 93 Next Smaller Element Solve the Next Smaller Element problem with a monotonic increasing stack in O(n). Python code, step-by-step trace, and reusable pattern for interviews. 6 min read →
14. 94 Heap Advanced Patterns Master advanced heap patterns -- merge K sorted lists, find median from data stream with two heaps, top K elements, task scheduler, and more with Python heapq implementations. 10 min read →
15. 95 Trap Rain Water (Stack) Solve Trapping Rain Water (LeetCode 42) using a stack-based approach. Python code with detailed trace, comparison with two-pointer, and complexity analysis. 6 min read →
16. 96 Score of Parentheses Solve Score of Parentheses using a stack to track nested scores. Python solution with trace, O(n) time, plus the bit-shift trick for O(1) space. 6 min read →
17. 97 Exclusive Time Functions Solve Exclusive Time of Functions with a stack simulating a call stack. Python solution with timestamps, detailed trace, and edge case handling. 6 min read →
18. 98 Flatten Nested List Design a Flatten Nested List Iterator using a stack for lazy flattening. Python solution with iterator protocol, step-by-step trace, and design analysis. 6 min read →
19. 99 Circular Deque Design a Circular Deque with front/rear pointers on a fixed-size array. Python solution with all O(1) operations, visual trace, and edge case handling. 6 min read →
20. 100 Heap Sort Understand heap sort, why build-heap is O(n) not O(n log n), sift-down vs sift-up, in-place sorting, partial sort for top K, and comparisons with other sorts. 12 min read →