BFS Pattern with Queues: Level-Order and Shortest Path
Master BFS using queues for tree level-order traversal and shortest path in unweighted graphs. Python implementations with detailed traces.
·3 min read · #dsa#queues#intermediate
#intermediate
442 posts · page 1 of 10
Design Circular Deque — Array-Based Implementation (LeetCode 641)
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 · #dsa#queues#design
Design Hit Counter Using Queue
Design a hit counter that counts hits in the past 5 minutes using a queue. LeetCode 362 solution with O(1) amortized operations.
·3 min read · #dsa#queues#intermediate
First Non-Repeating Character in a Stream
Find the first non-repeating character in a character stream using a queue and hash map. Python solution with O(1) amortized per query.
·3 min read · #dsa#queues#intermediate
Implement Stack Using Two Queues — Two Approaches Explained
Implement a stack using two queues with costly push and costly pop approaches. Complete Python solutions with complexity analysis.
·4 min read · #dsa#stacks#queues
Jump Game III and IV — BFS with Queue
Solve Jump Game III (LeetCode 1306) and Jump Game IV (LeetCode 1345) using BFS. Covers graph modeling of array problems with queue-based traversal.
·4 min read · #dsa#queues#intermediate
Rotten Oranges — Multi-Source BFS with Queue
Solve the Rotten Oranges problem (LeetCode 994) using multi-source BFS. Covers the simultaneous spread pattern, Python code, and grid BFS template.
·4 min read · #dsa#queues#intermediate
Shortest Path in Binary Matrix — BFS Solution
Find the shortest path in a binary matrix using BFS with 8-directional movement. LeetCode 1091 solution with Python code and grid traversal tips.
·3 min read · #dsa#queues#intermediate
Walls and Gates — Multi-Source BFS
Fill each empty room with the distance to its nearest gate using multi-source BFS. LeetCode 286 solution with Python code and grid BFS template.
·3 min read · #dsa#queues#intermediate
Snakes and Ladders BFS — Shortest Path on Game Board (LeetCode 909)
Solve Snakes and Ladders with BFS to find minimum dice rolls. Python solution with 1D-2D conversion, boustrophedon layout, and step-by-step trace.
·7 min read · #dsa#queues#bfs
Asteroid Collision Problem Using Stacks
Solve the asteroid collision problem (LeetCode 735) using a stack. Covers collision rules, Python implementation, and all edge cases.
·3 min read · #dsa#stacks#intermediate
Car Fleet Problem — Stack-Based Arrival Time Solution
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 · #dsa#stacks#sorting
The Celebrity Problem Using Stack-Based Elimination
Solve the celebrity problem in O(n) time using a stack elimination technique. Includes proof of correctness, Python code, and matrix examples.
·4 min read · #dsa#stacks#intermediate
Decode String (LeetCode 394) — Nested Bracket Decoding
Decode nested encoded strings like '3[a2[c]]' using a stack. Complete walkthrough with Python solution, traces, and edge cases.
·3 min read · #dsa#stacks#intermediate
Evaluate Reverse Polish Notation — Stack Solution Explained
Solve LeetCode 150 Evaluate Reverse Polish Notation using a stack. Python implementation with division gotcha, traces, and complexity analysis.
·5 min read · #dsa#stacks#intermediate
Exclusive Time of Functions — Call Stack Simulation (LeetCode 636)
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 · #dsa#stacks#intermediate
Implement Queue Using Two Stacks — Amortized O(1) Solution
Implement a queue using two stacks with amortized O(1) operations. Covers costly enqueue vs costly dequeue approaches with Python code.
·5 min read · #dsa#stacks#queues
Flatten Nested List Iterator — Lazy Stack Design (LeetCode 341)
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 · #dsa#stacks#design
Minimum Remove to Make Valid Parentheses — Stack Solution
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 · #dsa#stacks#strings
Online Stock Span — Class Design with Stack
Design an Online Stock Span class (LeetCode 901) using a stack. Covers amortized analysis, Python implementation, and the price-span pair technique.
·3 min read · #dsa#stacks#intermediate
Next Smaller Element Using Stack — Monotonic Increasing Stack Pattern
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 · #dsa#stacks#intermediate
Detect Redundant Parentheses Using a Stack
Learn to detect redundant parentheses in expressions using a stack. Covers the algorithm, Python implementation, and edge cases with traces.
·3 min read · #dsa#stacks#intermediate
Real-World Applications of Stacks and Queues — From Undo/Redo to BFS Crawlers
Real-world applications of stacks and queues: undo/redo systems, browser history, call stacks, task scheduling, message queues, and BFS web crawlers with Python examples.
·7 min read · #dsa#stacks#queues
Remove K Digits to Make Smallest Number
Use a monotonic stack to remove k digits from a number to make it as small as possible. LeetCode 402 solution with Python code and traces.
·4 min read · #dsa#stacks#intermediate
Simplify Unix Path Using a Stack — LeetCode 71 Solution
Solve Simplify Path LeetCode 71 with a stack. Handle ., .., multiple slashes, and edge cases. Python solution with step-by-step trace.
·4 min read · #dsa#stacks#strings
Score of Parentheses — Stack-Based Scoring (LeetCode 856)
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 · #dsa#stacks#intermediate
Sort a Stack Using Another Stack
Learn how to sort a stack using only one additional stack. Step-by-step trace, Python implementation, and O(n²) complexity analysis.
·3 min read · #dsa#stacks#intermediate
Stock Span Problem Using Stacks
Solve the stock span problem efficiently using a monotonic stack. Includes brute force vs optimal approach, Python code, and visual trace.
·4 min read · #dsa#stacks#intermediate
Validate Stack Sequences — Simulate Push/Pop (LeetCode 946)
Validate Stack Sequences solved by simulating push and pop operations. Python solution with step-by-step trace, edge cases, and complexity analysis for LeetCode 946.
·6 min read · #dsa#stacks#simulation
Binary Search on Answer: Advanced Problems
Solve advanced binary search on answer problems — Koko eating bananas, ship packages, split array largest sum, minimize max distance to gas station, and magnetic force between balls.
·13 min read · #dsa#binary-search#intermediate
Flatten Binary Tree to Linked List
Learn how to flatten a binary tree to a linked list using preorder threading, Morris traversal, and how to convert a BST to a sorted doubly linked list — with full Python implementations and Big-O analysis.
·9 min read · #dsa#trees#intermediate
BST Iterator, Range Queries, and Closest Value
Master BST iterator using stack-based controlled in-order traversal, range sum queries, counting nodes in range, closest value, and closest K values — with full Python implementations.
·11 min read · #dsa#trees#intermediate
Space Optimization Techniques in DSA
Master space optimization — rolling arrays for DP, in-place algorithms, bit manipulation as sets, Morris traversal, constant-space linked list operations, and Floyd's cycle detection.
·14 min read · #dsa#optimization#intermediate
Bipartite Graph Check: BFS 2-Coloring and DFS Approaches
Learn how to check if a graph is bipartite using BFS 2-coloring and DFS, with applications in matching, scheduling, and conflict detection.
·10 min read · #dsa#graphs#intermediate
Clone Graph, Valid Tree, and Graph Transformations
Learn to clone graphs with BFS and DFS, validate graph trees, find minimum height trees via centroid decomposition, and reconstruct itineraries with Hierholzer's algorithm.
·12 min read · #dsa#graphs#intermediate
Connected Components: DFS, BFS, and Union-Find Approaches
Master finding connected components using DFS, BFS, and Union-Find with applications to counting islands and grid connectivity problems.
·11 min read · #dsa#graphs#intermediate
Flood Fill, Surrounded Regions, and Enclaves
Master flood fill, surrounded regions, number of enclaves, and Pacific Atlantic water flow using DFS and BFS grid traversal techniques in Python.
·12 min read · #dsa#graphs#intermediate
Minimum Spanning Tree: Applications and Variants
Explore MST applications in network design, clustering, and competitive programming with second-best MST, critical edges, and minimum cost to connect points.
·11 min read · #dsa#graphs#intermediate
Shortest Path in Grid: BFS, 0-1 BFS, and Dijkstra on Grids
Master shortest path algorithms on grids including BFS for unweighted grids, 0-1 BFS with deque, and Dijkstra for weighted terrain with obstacles and portals.
·12 min read · #dsa#graphs#intermediate
Add Two Numbers as Linked Lists
Learn to add two numbers represented as linked lists — both reverse order (LeetCode 2) and forward order (LeetCode 445). Covers carry handling, different-length lists, and Python implementations with Big-O analysis.
·9 min read · #dsa#linked-lists#intermediate
Copy Linked List with Random Pointer
Learn two approaches to deep copy a linked list with random pointers — HashMap O(n) space and the interleaving O(1) space technique. Step-by-step walkthroughs with Python code and Big-O analysis.
·12 min read · #dsa#linked-lists#intermediate
Flatten a Multilevel Linked List
Learn to flatten a multilevel doubly linked list (LeetCode 430) and flatten sorted linked lists. Covers iterative and recursive DFS approaches with Python implementations and Big-O analysis.
·11 min read · #dsa#linked-lists#intermediate
Linked List Palindrome Check: Three Approaches
Learn three ways to check if a linked list is a palindrome — stack-based O(n) space, reverse-second-half O(1) space, and recursive. Step-by-step walkthroughs with Python code and Big-O analysis.
·10 min read · #dsa#linked-lists#intermediate
Partition and Rearrange Linked Lists
Master linked list partitioning — partition around a value (LeetCode 86), odd-even rearrangement (LeetCode 328), segregate 0s/1s/2s, with full Python implementations and Big-O analysis.
·12 min read · #dsa#linked-lists#intermediate
Remove Nth Node From End of Linked List
Master the two-pointer gap technique to remove the nth node from the end in a single pass. Covers the dummy node trick, edge cases like removing the head and single-node lists, with Python code and Big-O analysis.
·9 min read · #dsa#linked-lists#intermediate
Reorder Linked List: L0→Ln→L1→Ln-1
Learn how to reorder a linked list by interleaving first and last nodes — find the middle, reverse the second half, and merge alternating. Full Python code, step-by-step walkthrough, and Big-O analysis.
·9 min read · #dsa#linked-lists#intermediate
Rotate and Swap Operations on Linked Lists
Master linked list rotation by K positions, swap nodes in pairs, and swap Kth nodes from both ends — full Python implementations, step-by-step traces, and Big-O analysis.
·13 min read · #dsa#linked-lists#intermediate
Advanced Prefix Sum: 2D, Difference Arrays, and Beyond
Master advanced prefix sum techniques — 2D prefix sums for submatrix queries, difference arrays for range updates in O(1), subarray sum divisible by K, XOR prefix, and more.
·13 min read · #dsa#arrays#intermediate