Parallelize the lock BFS safely
Company: Snapchat
Role: Software Engineer
Category: System Design
Difficulty: hard
Interview Round: Technical Screen
Follow-up: If multiple threads are available, how would you parallelize the BFS for the rotating-lock problem? Describe how to partition work across threads, maintain BFS level ordering if required, implement a thread-safe visited set and queues, prevent duplicate exploration, and evaluate trade-offs such as contention, memory overhead, and determinism.
Quick Answer: This question evaluates a candidate's ability to design and reason about concurrent algorithms, including parallel BFS, synchronization, thread-safe data structures, partitioning strategies, and correctness guarantees for shortest-path searches.