Design a single-machine LRU cache
Company: DoorDash
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: Medium
Interview Round: Technical Screen
Design an in-memory LRU cache for a single machine using a hash map and a doubly linked list to support O(
1) get and put. Explain how you handle capacity, eviction, and key updates. Then make your design thread-safe: discuss lock granularity (global lock vs. per-bucket/segmented locks), readers–writer locks, lock-free alternatives, and how you would prevent race conditions and ensure memory safety. Analyze time and space complexity and outline major pitfalls.
Quick Answer: This question evaluates a candidate's ability to design an efficient in-memory LRU cache and apply concurrency control, covering eviction semantics, capacity management, key updates, and synchronization for thread safety.
Simulate get and put for an LRU cache with fixed capacity.
Examples
Input: (2, [('put', 1, 1), ('put', 2, 2), ('get', 1), ('put', 3, 3), ('get', 2), ('get', 3)])
Expected Output: [1, -1, 3]
Explanation: Evicts least recently used.
Input: (0, [('put', 1, 1), ('get', 1)])
Expected Output: [-1]
Explanation: Zero capacity.
Input: (1, [('put', 1, 1), ('put', 1, 2), ('get', 1)])
Expected Output: [2]
Explanation: Update existing key.