# Vahalla
> Prepare for modern technical interviews. Free, community-driven.

Vahalla is a free platform for practicing technical interviews. It offers 241 coding problems and 28 system design challenges with instant feedback.

## Why Vahalla
- Problems build across multiple levels — start simple, end production-grade
- AI-enabled rounds: debug and extend real codebases, not just write from scratch
- System design practice with guided sections and AI evaluation
- Community-reported company tags so you know what to expect
- Free tier includes full problem access with no paywall on content

## Getting Started
1. Browse problems at https://vahalla.dev/problems
2. Pick a problem and solve it in the browser — no local setup needed
3. Run tests instantly to check your solution
4. Progress through levels as problems get harder
5. Try system design challenges at https://vahalla.dev/system-design

## Features
- Multi-level coding problems that build in difficulty
- AI-enabled coding rounds (debug + extend a codebase)
- System design practice with guided sections and AI evaluation
- Monaco-powered code editor with test runner
- Company tags reported by the community
- Problem creation with AI-assisted generation

## Coding Problems
- [721. Accounts Merge](https://vahalla.dev/problem/accounts-merge) — Medium — Graphs, DFS, Union Find
- [415. Add Strings](https://vahalla.dev/problem/add-strings) — Easy — Strings, Math, Simulation
- [2. Add Two Numbers](https://vahalla.dev/problem/add-two-numbers) — Medium — Linked Lists, Math, Recursion
- [AI-Powered Task Management](https://vahalla.dev/problem/ai-todo-list) — Medium — Design, Graph, Topological Sort, State Machine
- [Word Guessing Game](https://vahalla.dev/problem/ai-word-game) — Medium — AI-Enabled, Debugging, String, Design
- [Wordle Solver](https://vahalla.dev/problem/ai-wordle-solver) — Hard — AI-Enabled, Debugging, String, Algorithm, Optimization
- [Anagrammed indexOf (Databricks)](https://vahalla.dev/problem/anagrammed-indexof) — Medium
- [Find Anagrams in Dictionary](https://vahalla.dev/problem/anagrams-in-dictionary) — Easy — Hash Table, String, Sorting
- [Backtracking Combinations](https://vahalla.dev/problem/backtracking-combinations) — Medium — Backtracking, Recursion, String
- [Banking System](https://vahalla.dev/problem/banking-system) — Hard — Design, Banking, Transactions, Cashback
- [227. Basic Calculator II](https://vahalla.dev/problem/basic-calculator-ii) — Medium — Stack, Math, Strings
- [121. Best Time to Buy and Sell Stock](https://vahalla.dev/problem/best-time-to-buy-sell-stock) — Easy — Arrays, Dynamic Programming
- [BikeMap JSON Processor](https://vahalla.dev/problem/bikemap-json-processor) — Medium — JSON, API, Data Processing, HTTP
- [124. Binary Tree Maximum Path Sum](https://vahalla.dev/problem/binary-tree-max-path-sum) — Hard — Trees, DFS, Dynamic Programming
- [199. Binary Tree Right Side View](https://vahalla.dev/problem/binary-tree-right-side-view) — Medium — Trees, BFS, DFS
- [Cool String](https://vahalla.dev/problem/bloomberg-cool-string) — Medium — Hash Table, Greedy, Sorting, String
- [Count Islands by Requested Sizes](https://vahalla.dev/problem/bloomberg-count-islands) — Medium — BFS, Graph, Matrix, Hash Table
- [Deck of Cards Shuffling](https://vahalla.dev/problem/bloomberg-deck-shuffle) — Medium — Randomization, Simulation, Design
- [Flag Transactions](https://vahalla.dev/problem/bloomberg-flag-transactions) — Medium — Hash Table, Sorting, String, Fintech
- [Sparse Array & Matrix](https://vahalla.dev/problem/bloomberg-sparse-array) — Hard — Design, Array, Matrix, Hash Map, Math
- [Valid Frequency String Check](https://vahalla.dev/problem/bloomberg-valid-frequency) — Easy — Hash Table, String, Counting
- [Book Price Aggregator Platform (Databricks)](https://vahalla.dev/problem/book-price-aggregator) — Hard
- [Left and Right Side View of a BST](https://vahalla.dev/problem/bst-left-right-side-view) — Medium — Trees, BFS, Binary Search Tree
- [426. Convert BST to Sorted Doubly Linked List](https://vahalla.dev/problem/bst-to-doubly-linked-list) — Medium — Trees, DFS, Linked List
- [Bubble Sort](https://vahalla.dev/problem/bubble-sort) — Easy — Sorting, Arrays, Beginner
- [Buffered File Writer](https://vahalla.dev/problem/buffered-file-writer) — Medium — Concurrency, Systems, Buffering
- [1762. Buildings With an Ocean View](https://vahalla.dev/problem/buildings-ocean-view) — Medium — Arrays, Stack, Monotonic Stack
- [Campsite](https://vahalla.dev/problem/campsite) — Medium — BFS, Grid, Shortest Path
- [Carbon Footprint Aggregator](https://vahalla.dev/problem/carbon-footprint-aggregator) — Medium — Hash Map, Data Processing, Business Logic
- [Card Game Win Calculator](https://vahalla.dev/problem/card-game-wins) — Medium — Simulation, Sorting, Hash Table
- [Compute Cart Total with Best Promotion](https://vahalla.dev/problem/cart-promotions) — Medium — Algorithms, Greedy, Simulation
- [Chat Messages History](https://vahalla.dev/problem/chat-messages-history) — Hard — Design, Data Structures, TTL, Backup/Restore
- [IPv4 CIDR Firewall Processor](https://vahalla.dev/problem/cidr-firewall) — Hard — Bit Manipulation, Networking, Intervals, Algorithms
- [Circuit Breaker Implementation (Databricks)](https://vahalla.dev/problem/circuit-breaker) — Medium
- [Circular Task Distribution](https://vahalla.dev/problem/circular-distribution) — Medium
- [133. Clone Graph](https://vahalla.dev/problem/clone-graph) — Medium — Graphs, BFS, DFS, Hash Table
- [Cloud Storage System](https://vahalla.dev/problem/cloud-storage-system) — Hard — Data Structures, System Design, API Design
- [Cloud Storage System (V2)](https://vahalla.dev/problem/cloud-storage-system-v2) — Hard — Data Structures, System Design, API Design
- [Code Diff Generator](https://vahalla.dev/problem/code-diff-generator) — Hard — Dynamic Programming, LCS, String
- [Crypto Order System](https://vahalla.dev/problem/coinbase-crypto-order-system) — Hard — Hash Map, Design, State Machine
- [Currency Exchange Best Rate](https://vahalla.dev/problem/coinbase-currency-exchange) — Medium — Graph, BFS, DFS, Bellman-Ford
- [Generate Random NFT](https://vahalla.dev/problem/coinbase-generate-random-nft) — Medium — Hash Map, Randomization, Weighted Sampling
- [Mining Block - Fee Maximization (Knapsack)](https://vahalla.dev/problem/coinbase-mining-block-fee) — Medium — Dynamic Programming, Knapsack, Graph, DAG, Topological Sort
- [Order Management System (Kafka Consumer)](https://vahalla.dev/problem/coinbase-order-management) — Medium — Design, State Machine, Hash Map, Idempotency
- [Stream Metrics Analysis](https://vahalla.dev/problem/coinbase-stream-metrics) — Medium — Hash Map, Design, Streaming
- [Transaction Filter with Pagination](https://vahalla.dev/problem/coinbase-transaction-filter) — Medium — Design, Pagination, Sorting, Filtering, Cursor
- [Shell Command Parser](https://vahalla.dev/problem/command-parser) — Hard — Parsing, State Machine, String Processing, Shell
- [Concurrent Range-Aware File Caching Client](https://vahalla.dev/problem/concurrent-file-cache) — Hard — System Design, Caching, Concurrency, File Systems
- [219. Contains Duplicate II](https://vahalla.dev/problem/contains-duplicate-ii) — Easy — Arrays, Hash Table, Sliding Window
- [138. Copy List with Random Pointer](https://vahalla.dev/problem/copy-list-random-pointer) — Medium — Linked Lists, Hash Table
- [Counting Sort](https://vahalla.dev/problem/counting-sort) — Medium — Sorting, Non-comparison
- [207. Course Schedule](https://vahalla.dev/problem/course-schedule) — Medium — Graphs, Topological Sort, DFS, BFS
- [791. Custom Sort String](https://vahalla.dev/problem/custom-sort-string) — Medium — Hash Table, Strings, Sorting
- [Dasher Pay Calculation](https://vahalla.dev/problem/dasher-pay) — Medium — Algorithms, Sweep Line, Simulation, Intervals
- [394. Decode String](https://vahalla.dev/problem/decode-string) — Medium — Strings, Stack, Recursion
- [Package Dependency Validator](https://vahalla.dev/problem/dependency-validator) — Hard — Graph, DFS, Version Comparison, Topological Sort
- [498. Diagonal Traverse](https://vahalla.dev/problem/diagonal-traverse) — Medium — Array, Matrix, Simulation
- [543. Diameter of Binary Tree](https://vahalla.dev/problem/diameter-binary-tree) — Easy — Trees, DFS, Recursion
- [Distributed File System (Databricks)](https://vahalla.dev/problem/distributed-file-system) — Hard
- [Distributed Document Search Index](https://vahalla.dev/problem/document-search-index) — Hard — Design, Hash Map, Set Operations, Search
- [Duplicate Files](https://vahalla.dev/problem/duplicate-files) — Medium — Hash Map, File System, OS
- [Durable Log Writer (Databricks)](https://vahalla.dev/problem/durable-log-writer) — Hard
- [Employee Management System](https://vahalla.dev/problem/employee-management) — Hard — Design, OOP, Interval, Heap
- [Encode String](https://vahalla.dev/problem/encode-string) — Medium — String, Hash Map, Encoding
- [Production ETL Pipeline](https://vahalla.dev/problem/etl-pipeline) — Medium — API, Databases, Error Handling, ETL, CLI
- [636. Exclusive Time of Functions](https://vahalla.dev/problem/exclusive-time-functions) — Medium — Stack, Arrays
- [Find Path in Implicit Fibonacci Tree](https://vahalla.dev/problem/fibonacci-tree-path) — Hard — Tree, Recursion, Binary Tree, Math, Fibonacci
- [Large-Scale File Deduplicator](https://vahalla.dev/problem/file-deduplicator) — Hard — Hashing, File I/O, Systems, Algorithms
- [File Storage System](https://vahalla.dev/problem/file-storage) — Hard — Design, File System, Versioning, Tree
- [34. Find First and Last Position](https://vahalla.dev/problem/find-first-last-position) — Medium — Binary Search, Arrays
- [295. Find Median from Data Stream](https://vahalla.dev/problem/find-median-data-stream) — Hard — Heap, Design, Data Stream
- [162. Find Peak Element](https://vahalla.dev/problem/find-peak-element) — Medium — Binary Search, Arrays
- [Find Points in 2D Plane](https://vahalla.dev/problem/find-points) — Hard — Binary Search, Divide and Conquer, 2D Search
- [Formula Evaluation](https://vahalla.dev/problem/formula-evaluation) — Medium — Graph, Topological Sort, Hash Map, Parsing
- [22. Generate Parentheses](https://vahalla.dev/problem/generate-parentheses) — Medium — Backtracking, Strings, Dynamic Programming
- [824. Goat Latin](https://vahalla.dev/problem/goat-latin) — Easy — Strings
- [GPU Quota Management System](https://vahalla.dev/problem/gpu-quota-management) — Hard — Design, Heap, Simulation, Time-based
- [49. Group Anagrams](https://vahalla.dev/problem/group-anagrams) — Medium — Strings, Hash Table, Sorting
- [Group Coordinates](https://vahalla.dev/problem/group-coordinates) — Medium — Union Find, Graph, BFS, DFS
- [249. Group Shifted Strings](https://vahalla.dev/problem/group-shifted-strings) — Medium — Strings, Hash Table
- [Heap Sort](https://vahalla.dev/problem/heap-sort) — Medium — Sorting, Heap, Binary Tree
- [Hierarchical Access Control System](https://vahalla.dev/problem/hierarchical-access-control) — Medium — Tree, DFS, Hash Table, Design
- [House Robber Variants (Databricks)](https://vahalla.dev/problem/house-robber-variants) — Medium
- [In-Memory Cache with Write-Ahead Log (Databricks)](https://vahalla.dev/problem/in-memory-cache-wal) — Hard
- [In-Memory Database](https://vahalla.dev/problem/in-memory-database) — Hard — Data Structures, System Design, API Design, TTL, Backup
- [94. Binary Tree Inorder Traversal](https://vahalla.dev/problem/inorder-traversal) — Easy — Trees, DFS, Stack
- [708. Insert into a Sorted Circular Linked List](https://vahalla.dev/problem/insert-sorted-circular) — Medium — Linked Lists
- [Insertion Sort](https://vahalla.dev/problem/insertion-sort) — Easy — Sorting, Arrays, Beginner
- [986. Interval List Intersections](https://vahalla.dev/problem/interval-intersections) — Medium — Arrays, Two Pointers
- [Inventory Management System](https://vahalla.dev/problem/inventory-management-system) — Hard — Design, Data Structures, String Processing, API Design
- [226. Invert Binary Tree](https://vahalla.dev/problem/invert-binary-tree) — Easy — Trees, DFS, BFS, Recursion
- [Job Monitor](https://vahalla.dev/problem/job-monitor) — Medium — Hash Map, Design, Simulation
- [Joker Poker](https://vahalla.dev/problem/joker-poker) — Medium — Design, Hash Map, Sorting, OOP
- [973. K Closest Points to Origin](https://vahalla.dev/problem/k-closest-points) — Medium — Heap, Sorting, Quickselect
- [Key Value Store](https://vahalla.dev/problem/key-value-store) — Hard — Design, Stack, Hash Map, Transactions
- [215. Kth Largest Element in an Array](https://vahalla.dev/problem/kth-largest-element) — Medium — Heap, Quickselect, Sorting
- [1539. Kth Missing Positive Number](https://vahalla.dev/problem/kth-missing-positive) — Easy — Array, Binary Search
- [230. Kth Smallest Element in a BST](https://vahalla.dev/problem/kth-smallest-bst) — Medium — Trees, DFS, BST, Inorder
- [Design KV Store with Sliding-Window Average QPS](https://vahalla.dev/problem/kv-store-qps) — Medium — Design, Hash Table, Sliding Window, Metrics, Generics
- [Lane Segment](https://vahalla.dev/problem/lane-segment) — Hard — Geometry, Union Find, Sorting
- [Largest Connected Component](https://vahalla.dev/problem/largest-component) — Medium
- [LazyArray with Deferred Execution (Databricks)](https://vahalla.dev/problem/lazy-array) — Medium
- [102. Binary Tree Level Order Traversal](https://vahalla.dev/problem/level-order-traversal) — Medium — Trees, BFS, Queue
- [Round-Robin Load Balancer](https://vahalla.dev/problem/load-balancer) — Medium — Design, Data Structures, Linked List
- [Lock Optimization and Deadlock Detection (Databricks)](https://vahalla.dev/problem/lock-deadlock-detection) — Hard
- [14. Longest Common Prefix](https://vahalla.dev/problem/longest-common-prefix) — Easy — Strings
- [Find Longest Common Ordered Restaurant List](https://vahalla.dev/problem/longest-common-restaurants) — Hard — Dynamic Programming, Algorithms, Strings
- [5. Longest Palindromic Substring](https://vahalla.dev/problem/longest-palindromic-substring) — Medium — Strings, Dynamic Programming, Two Pointers
- [3. Longest Substring Without Repeating Characters](https://vahalla.dev/problem/longest-substring-no-repeat) — Medium — Strings, Sliding Window, Hash Table
- [236. Lowest Common Ancestor of a Binary Tree](https://vahalla.dev/problem/lowest-common-ancestor) — Medium — Trees, DFS, Recursion
- [146. LRU Cache](https://vahalla.dev/problem/lru-cache) — Medium — Design, Hash Table, Linked List
- [169. Majority Element](https://vahalla.dev/problem/majority-element) — Easy — Arrays, Boyer-Moore Voting
- [The '1TB Text File'](https://vahalla.dev/problem/massive-text-file) — Hard — File I/O, Indexing, Systems, Memory Management
- [Matrix Layer Rotation](https://vahalla.dev/problem/matrix-layer-rotation) — Hard — Array, Matrix, Simulation
- [1004. Max Consecutive Ones III](https://vahalla.dev/problem/max-consecutive-ones-iii) — Medium — Arrays, Sliding Window
- [104. Maximum Depth of Binary Tree](https://vahalla.dev/problem/max-depth-binary-tree) — Easy — Trees, DFS, BFS, Recursion
- [Maximum Difference Tracker](https://vahalla.dev/problem/max-diff-tracker) — Easy
- [Maximal Square / Maximal Rectangle (Databricks)](https://vahalla.dev/problem/maximal-square-rectangle) — Medium
- [53. Maximum Subarray](https://vahalla.dev/problem/maximum-subarray) — Medium — Arrays, Dynamic Programming, Kadane's Algorithm
- [670. Maximum Swap](https://vahalla.dev/problem/maximum-swap) — Medium — Math, Greedy, Strings
- [Maze Game](https://vahalla.dev/problem/maze-game) — Medium — BFS, OOP, Game Logic, Debugging, AI Enabled
- [Maze Solver with Path Printing](https://vahalla.dev/problem/maze-solver) — Medium — BFS, Graph, Matrix, Debugging, AI Enabled
- [4. Median of Two Sorted Arrays](https://vahalla.dev/problem/median-two-sorted-arrays) — Hard — Binary Search, Arrays, Divide and Conquer
- [253. Meeting Rooms II](https://vahalla.dev/problem/meeting-rooms-ii) — Medium — Arrays, Heap, Sorting
- [56. Merge Intervals](https://vahalla.dev/problem/merge-intervals) — Medium — Arrays, Sorting
- [23. Merge k Sorted Lists](https://vahalla.dev/problem/merge-k-sorted-lists) — Hard — Linked Lists, Heap, Divide and Conquer
- [Merge Sort](https://vahalla.dev/problem/merge-sort) — Medium — Sorting, Divide and Conquer, Recursion
- [88. Merge Sorted Array](https://vahalla.dev/problem/merge-sorted-array) — Easy — Arrays, Two Pointers, Sorting
- [21. Merge Two Sorted Lists](https://vahalla.dev/problem/merge-two-sorted-lists) — Easy — Linked Lists, Recursion
- [Merkle Tree Directory Diff](https://vahalla.dev/problem/merkle-tree-diff) — Hard — Tree, Hash, File System, Recursion
- [Message Parser](https://vahalla.dev/problem/message-parser) — Medium — Parsing, Hash Map, Recursion
- [Minesweeper](https://vahalla.dev/problem/minesweeper) — Medium — BFS, DFS, Matrix, Game
- [921. Minimum Add to Make Parentheses Valid](https://vahalla.dev/problem/minimum-add-valid-parentheses) — Medium — Strings, Stack, Greedy
- [1249. Minimum Remove to Make Valid Parentheses](https://vahalla.dev/problem/minimum-remove-valid-parentheses) — Medium — Strings, Stack
- [Get Minimum Round Trip Cost](https://vahalla.dev/problem/minimum-round-trip-cost) — Easy — Array, Greedy, Prefix
- [76. Minimum Window Substring](https://vahalla.dev/problem/minimum-window-substring) — Hard — Strings, Sliding Window, Hash Table
- [163. Missing Ranges](https://vahalla.dev/problem/missing-ranges) — Easy — Arrays, Strings
- [Help Mouse Find the Cheese](https://vahalla.dev/problem/mouse-find-cheese) — Medium — DFS, Backtracking, Matrix, Graph
- [283. Move Zeroes](https://vahalla.dev/problem/move-zeroes) — Easy — Arrays, Two Pointers
- [Movie Recommendation System](https://vahalla.dev/problem/movie-recommendations) — Hard — Intervals, Set Operations, Algorithm Design
- [Multi-Modal City Navigation](https://vahalla.dev/problem/multi-modal-city) — Medium — BFS, Grid, Shortest Path
- [Multi-threaded Chat System (Databricks)](https://vahalla.dev/problem/multithreaded-chat) — Hard
- [339. Nested List Weight Sum](https://vahalla.dev/problem/nested-list-weight-sum) — Easy — DFS, Recursion, Stack
- [Network Throttling System (Databricks)](https://vahalla.dev/problem/network-throttling) — Hard
- [31. Next Permutation](https://vahalla.dev/problem/next-permutation) — Medium — Arrays, Two Pointers
- [200. Number of Islands](https://vahalla.dev/problem/number-of-islands) — Medium — Graphs, BFS, DFS, Union Find
- [OAuth Data Migration Client](https://vahalla.dev/problem/oauth-data-migration) — Medium — API, OAuth, ETL, Pagination, Error Handling, W&B
- [9. Palindrome Number](https://vahalla.dev/problem/palindrome-number) — Easy — Math, Strings
- [647. Palindromic Substrings](https://vahalla.dev/problem/palindromic-substrings) — Medium — Strings, Dynamic Programming, Two Pointers
- [Perplexity FDE 1: Basic API Client](https://vahalla.dev/problem/perplexity-fde-1) — Medium — API, HTTP, Authentication
- [Perplexity FDE 2: Citation Extraction](https://vahalla.dev/problem/perplexity-fde-2) — Medium — API, Parsing, Data Processing
- [Perplexity FDE 3: Error Handling & Retries](https://vahalla.dev/problem/perplexity-fde-3) — Medium — API, Error Handling, Retry Logic
- [Perplexity FDE 4: Streaming Responses](https://vahalla.dev/problem/perplexity-fde-4) — Hard — API, Streaming, SSE, Generators
- [Perplexity FDE 5: Edge Cases](https://vahalla.dev/problem/perplexity-fde-5) — Hard — API, Validation, Edge Cases, Security
- [Perplexity FDE 6: Metrics & Observability](https://vahalla.dev/problem/perplexity-fde-6) — Hard — API, Metrics, Observability, Production
- [Bank ID Resolution](https://vahalla.dev/problem/plaid-bank-id-resolution) — Medium — Hash Table, Design, Data Processing
- [Coupon](https://vahalla.dev/problem/plaid-coupon) — Medium — Design, Greedy, Validation
- [Coupon II](https://vahalla.dev/problem/plaid-coupon-ii) — Medium — Design, Greedy, Validation
- [Group Transactions](https://vahalla.dev/problem/plaid-group-transactions) — Easy — Hash Table, Data Processing
- [Hiring Manager Behavioral Interview](https://vahalla.dev/problem/plaid-hm-behavioral) — Medium — Behavioral, Hiring Manager, Leadership
- [Payment Schedule](https://vahalla.dev/problem/plaid-payment-schedule) — Medium — Hash Table, Sorting, Data Processing
- [Rate Limiter](https://vahalla.dev/problem/plaid-rate-limiter) — Medium — Design, Queue, Sliding Window
- [Settlement](https://vahalla.dev/problem/plaid-settlement) — Medium — Hash Table, String Parsing, Design
- [50. Pow(x, n)](https://vahalla.dev/problem/pow-x-n) — Medium — Math, Recursion, Binary Exponentiation
- [238. Product of Array Except Self](https://vahalla.dev/problem/product-except-self) — Medium — Arrays, Prefix Sum
- [Contextual Prompt Scorer](https://vahalla.dev/problem/prompt-feature-scorer) — Medium — String Matching, Scoring, NLP, Feature Extraction
- [Quick Sort](https://vahalla.dev/problem/quick-sort) — Medium — Sorting, Divide and Conquer, Recursion
- [Radix Sort](https://vahalla.dev/problem/radix-sort) — Medium — Sorting, Non-comparison
- [398. Random Pick Index](https://vahalla.dev/problem/random-pick-index) — Medium — Hash Table, Reservoir Sampling, Random
- [528. Random Pick with Weight](https://vahalla.dev/problem/random-pick-with-weight) — Medium — Prefix Sum, Binary Search, Random
- [Range Query](https://vahalla.dev/problem/range-query) — Medium — Binary Search, Array
- [938. Range Sum of BST](https://vahalla.dev/problem/range-sum-bst) — Easy — Trees, DFS, BST
- [Rate-Limited Hit Counter](https://vahalla.dev/problem/rate-limited-hit-counter) — Medium — Design, Queue, Data Stream, Sliding Window, Concurrency
- [Rate Limiter](https://vahalla.dev/problem/rate-limiter) — Hard — Design, Rate Limiting, Sliding Window, System Design
- [Real-time Chat System (Slack) (Databricks)](https://vahalla.dev/problem/realtime-chat-system) — Hard
- [Recipe Manager](https://vahalla.dev/problem/recipe-manager) — Hard — Design, CRUD, Search, Versioning
- [Relative Positions by Greater Elements](https://vahalla.dev/problem/relative-positions) — Medium
- [1047. Remove All Adjacent Duplicates In String](https://vahalla.dev/problem/remove-adjacent-duplicates) — Easy — Strings, Stack
- [26. Remove Duplicates from Sorted Array](https://vahalla.dev/problem/remove-duplicates) — Easy — Arrays, Two Pointers
- [19. Remove Nth Node From End of List](https://vahalla.dev/problem/remove-nth-from-end) — Medium — Linked Lists, Two Pointers
- [Restaurant Waiter Path](https://vahalla.dev/problem/restaurant-waiter-path) — Medium — BFS, Grid, Shortest Path, Bitmask DP
- [206. Reverse Linked List](https://vahalla.dev/problem/reverse-linked-list) — Easy — Linked Lists, Recursion
- [Run-Length Encoded Bitwise AND](https://vahalla.dev/problem/rle-bitwise-and) — Medium — Bit Manipulation, Two Pointers, Run-Length Encoding
- [Party Time - Party Windows and Dead Zones](https://vahalla.dev/problem/scale-party-time) — Medium — Hash Map, Sorting, Intervals, Data Processing
- [33. Search in Rotated Sorted Array](https://vahalla.dev/problem/search-rotated-array) — Medium — Binary Search, Arrays
- [Selection Sort](https://vahalla.dev/problem/selection-sort) — Easy — Sorting, Arrays, Beginner
- [1091. Shortest Path in Binary Matrix](https://vahalla.dev/problem/shortest-path-binary-matrix) — Medium — Graphs, BFS, Matrix
- [Simple Task Management System](https://vahalla.dev/problem/simple-task-management-system) — Hard — Design, Implementation, Data Structures, History Tracking
- [71. Simplify Path](https://vahalla.dev/problem/simplify-path) — Medium — Stack, Strings
- [239. Sliding Window Maximum](https://vahalla.dev/problem/sliding-window-maximum) — Hard — Deque, Sliding Window, Monotonic Queue
- [Sorting Algorithm Comparison](https://vahalla.dev/problem/sorting-comparison) — Easy — Sorting, Summary
- [Spaceship Navigation Through Gravitational Fields](https://vahalla.dev/problem/spaceship-gaps) — Medium — Intervals, Merge Intervals, Array
- [1570. Dot Product of Two Sparse Vectors](https://vahalla.dev/problem/sparse-vector-dot-product) — Medium — Hash Table, Design, Arrays
- [SQL Query Optimizer - Predicate Pushdown (Databricks)](https://vahalla.dev/problem/sql-query-optimizer) — Hard
- [Stock Portfolio Tracking](https://vahalla.dev/problem/stock-portfolio) — Medium — Hash Table, Design, Simulation
- [8. String to Integer (atoi)](https://vahalla.dev/problem/string-to-integer) — Medium — Strings, Parsing
- [CSV Company Data Verification](https://vahalla.dev/problem/stripe-csv-verification) — Medium — String Processing, Design, OOD, CSV Parsing
- [Student Course & Grade Management](https://vahalla.dev/problem/student-course-management) — Hard — Design, Data Structures, API Design, Sorting
- [560. Subarray Sum Equals K](https://vahalla.dev/problem/subarray-sum-equals-k) — Medium — Arrays, Hash Table, Prefix Sum
- [78. Subsets](https://vahalla.dev/problem/subsets) — Medium — Backtracking, Arrays, Bit Manipulation
- [572. Subtree of Another Tree](https://vahalla.dev/problem/subtree-of-another-tree) — Easy — Trees, DFS, Recursion
- [129. Sum Root to Leaf Numbers](https://vahalla.dev/problem/sum-root-to-leaf) — Medium — Trees, DFS
- [Minimum Interval to Cover Range](https://vahalla.dev/problem/suno-min-interval-cover) — Medium — Greedy, Sorting, Intervals
- [Minimum Interval to Include Each Query](https://vahalla.dev/problem/suno-min-interval-query) — Hard — Sorting, Heap, Intervals
- [Minimum Number of Taps to Water a Garden](https://vahalla.dev/problem/suno-min-taps) — Hard — Greedy, Intervals, Dynamic Programming
- [Referral Credits](https://vahalla.dev/problem/suno-referral-credits) — Medium — Hash Table, Data Processing, Design
- [Hyperparameter Sweep Coordinator](https://vahalla.dev/problem/sweep-coordinator) — Hard — ML Infrastructure, Hyperparameter Tuning, Distributed Systems, W&B Sweeps
- [Swipe Left - Merge Numbers](https://vahalla.dev/problem/swipe-left) — Medium — Arrays, Simulation, 2048 Game
- [Task Management System](https://vahalla.dev/problem/task-management-system) — Hard — Design, Hash Map, Sorting
- [Multi-Level Task Manager](https://vahalla.dev/problem/task-manager) — Hard — Design, Task Management, Sorting, Time Windows
- [Task Scheduler](https://vahalla.dev/problem/task-scheduler) — Hard — Design, Queue, Priority Queue, DAG, Dependencies
- [Thread-Safe Bounded MPMC Queue](https://vahalla.dev/problem/thread-safe-mpmc-queue) — Hard — Concurrency, Threading, Queue, Design
- [15. 3Sum](https://vahalla.dev/problem/three-sum) — Medium — Arrays, Two Pointers, Sorting
- [Design Tic-Tac-Toe and QPS Data Structure](https://vahalla.dev/problem/tic-tac-toe-qps) — Medium — Design, Matrix, Game Theory, Data Stream
- [Time Series Data Imputation](https://vahalla.dev/problem/time-series-imputation) — Medium — Array, Data Processing, Interpolation
- [766. Toeplitz Matrix](https://vahalla.dev/problem/toeplitz-matrix) — Easy — Arrays, Matrix
- [Top Active Users by Session Time](https://vahalla.dev/problem/top-active-users) — Medium — Hash Table, Sorting, DateTime, Data Processing
- [347. Top K Frequent Elements](https://vahalla.dev/problem/top-k-frequent) — Medium — Hash Table, Heap, Bucket Sort
- [42. Trapping Rain Water](https://vahalla.dev/problem/trapping-rain-water) — Hard — Arrays, Two Pointers, Dynamic Programming, Stack
- [TV Show User Retention Analysis](https://vahalla.dev/problem/tv-show-retention) — Easy — Math, Simulation, Arrays
- [1. Two Sum](https://vahalla.dev/problem/two-sum) — Easy — Arrays, Hash Table
- [UART Handler](https://vahalla.dev/problem/uart-handler) — Medium — Queue, Embedded Systems, Protocol Parsing
- [Ultimate Tic Tac Toe](https://vahalla.dev/problem/ultimate-tic-tac-toe) — Hard — Game, OOP, Matrix, State Machine
- [Stable De-duplication](https://vahalla.dev/problem/unique-stable) — Medium — Generator, Hashing, Stream Processing
- [Unix Cd Command with Symbolic Link Resolution](https://vahalla.dev/problem/unix-cd-command) — Medium — Stack, String, DFS, Simulation
- [242. Valid Anagram](https://vahalla.dev/problem/valid-anagram) — Easy — Strings, Hash Table, Sorting
- [65. Valid Number](https://vahalla.dev/problem/valid-number) — Hard — Strings, State Machine
- [125. Valid Palindrome](https://vahalla.dev/problem/valid-palindrome) — Easy — Strings, Two Pointers
- [680. Valid Palindrome II](https://vahalla.dev/problem/valid-palindrome-ii) — Easy — Strings, Two Pointers, Greedy
- [20. Valid Parentheses](https://vahalla.dev/problem/valid-parentheses) — Easy — Stack, Strings
- [408. Valid Word Abbreviation](https://vahalla.dev/problem/valid-word-abbreviation) — Easy — Strings, Two Pointers
- [98. Validate Binary Search Tree](https://vahalla.dev/problem/validate-bst) — Medium — Trees, DFS, BST
- [Vehicle Collision Simulation](https://vahalla.dev/problem/vehicle-collision) — Hard — Simulation, Geometry, Physics
- [Vehicle Velocity Log](https://vahalla.dev/problem/vehicle-velocity-log) — Medium — Hash Map, Statistics, Sliding Window
- [Concurrent Web Crawler](https://vahalla.dev/problem/web-crawler) — Hard — Design, BFS, Concurrency, Rate Limiting, Retry
- [Windowed Average with TTL](https://vahalla.dev/problem/windowed-average) — Hard — Design, Doubly Linked List, Hash Map, Sliding Window
- [Word Distance Matching](https://vahalla.dev/problem/word-distance-matching) — Medium — Arrays, Hash Table, String Matching
- [Word Distance Matching (Ordered)](https://vahalla.dev/problem/word-distance-matching-ordered) — Medium — String, Array, Two Pointers
- [127. Word Ladder](https://vahalla.dev/problem/word-ladder) — Hard — Graphs, BFS, Hash Table
- [Working Hours Register](https://vahalla.dev/problem/working-hours-register) — Hard — Design, Implementation, Intervals
- [The 'Zero-Copy' DataLoader](https://vahalla.dev/problem/zero-copy-dataloader) — Hard — Concurrency, Low-level, Multiprocessing, Shared Memory

## System Design Problems
- [Design an Ad Click Aggregator](https://vahalla.dev/system-design/ad-click-aggregator) — Hard
- [Design Coinbase Explore (Real-time Crypto Prices)](https://vahalla.dev/system-design/coinbase-explore) — Hard
- [Design Discord](https://vahalla.dev/system-design/discord) — Hard
- [Design DoorDash](https://vahalla.dev/system-design/doordash) — Hard
- [Design Dropbox](https://vahalla.dev/system-design/dropbox) — Hard
- [Design Google Docs](https://vahalla.dev/system-design/google-docs) — Hard
- [Design Google Maps](https://vahalla.dev/system-design/google-maps) — Hard
- [Design Instagram](https://vahalla.dev/system-design/instagram) — Hard
- [Design a Distributed Job Scheduler](https://vahalla.dev/system-design/job-scheduler) — Hard
- [Design LeetCode](https://vahalla.dev/system-design/leetcode) — Medium
- [Design a Multi-Tenant LLM Inference Platform](https://vahalla.dev/system-design/llm-inference-platform) — Hard
- [Design a Social Media News Feed](https://vahalla.dev/system-design/news-feed) — Hard
- [Design a Notification System](https://vahalla.dev/system-design/notification-system) — Medium
- [Design an Online Auction System](https://vahalla.dev/system-design/online-auction) — Medium
- [Design a Payment System](https://vahalla.dev/system-design/payment-system) — Hard
- [Design a Rate Limiter](https://vahalla.dev/system-design/rate-limiter) — Medium
- [Design Robinhood](https://vahalla.dev/system-design/robinhood) — Hard
- [Design Spotify](https://vahalla.dev/system-design/spotify) — Hard
- [Design Ticketmaster](https://vahalla.dev/system-design/ticketmaster) — Hard
- [Design Tinder](https://vahalla.dev/system-design/tinder) — Medium
- [Design a Top-K System](https://vahalla.dev/system-design/top-k) — Medium
- [Design Twitter Search](https://vahalla.dev/system-design/twitter-search) — Hard
- [Design Uber](https://vahalla.dev/system-design/uber) — Hard
- [Design a URL Shortener (Bitly)](https://vahalla.dev/system-design/url-shortener) — Medium
- [Design a Web Crawler](https://vahalla.dev/system-design/web-crawler) — Hard
- [Design WhatsApp](https://vahalla.dev/system-design/whatsapp) — Hard
- [Design Yelp](https://vahalla.dev/system-design/yelp) — Medium
- [Design YouTube](https://vahalla.dev/system-design/youtube) — Hard

## Links
- Homepage: https://vahalla.dev
- Pricing: https://vahalla.dev/pricing
- Full content for LLMs: https://vahalla.dev/llms-full.txt
- Terms: https://vahalla.dev/terms
- Contact: https://vahalla.dev/contact