We'll never share your email with anyone else.
No spam we promise.
Data Labeling Allocator
Type Interpreter
Design CI/CD Pipeline
Counting Subsequences
[System Design] Job Scheduler Part 1
Microorganism Simulation[OA 2026]
Several microorganisms of different families have been arranged next to each other. Write a program that returns the largest family...
Count Colorful Paths [OA 2026]
You are given an undirected graph with n nodes. Return the number of colorful paths...
Find Network Endpoint [OA 2026]
Given a directed graph, starting from node start, follow the edges and return the last node reached...

Guess Secret Number
Data Labeling Allocator II
Code Refactoring - Chat Agent Handler
Message Tree Delivery
Implement the 'cd' Command
[System Design] Build Job Orchestration System
> 2026 > OpenAI > Coding
Social Network with Snapshots
Design an in-memory social network from the ground up. Start with a directed follow graph, then layer on immutable snapshots, timestamped "time travel" queries, and a friends-of-friends recommendation engine.
Four levels
Support follow, unfollow, and look up a user's followers and followees. Snapshots stay frozen as the graph changes, and is_following_at(a, b, t) answers whether a followed b at time t.
Read the full problem and solution →

Crossword Solver
Shard Rebalancing in KV Store
Persistent KV Store
Design and Implement DB
Design and implement Resumable Iterator
Serialize Cluster Topology
Serialize and restore a cluster topology while preserving every node and connection.
Grid Infection
Simulate an infection spreading across a grid and determine when every reachable cell is infected.