Design a Phone Directory (Linked List/Array)
Design a phone directory that allows users to reserve and release numbers. Implement using a linked list or array combined with efficient tracking of available numbers.
Why Interviewers Ask This
Stripe evaluates candidates on their ability to balance memory efficiency with constant-time operations in resource-constrained environments. This question tests your understanding of trade-offs between array-based indexing for O(1) access versus linked list flexibility, specifically focusing on how to manage sparse data sets like phone numbers without wasting excessive memory while ensuring fast reservation and release cycles.
How to Answer This Question
1. Clarify requirements immediately: Ask about the range of phone numbers (sparse vs. dense), expected volume of concurrent requests, and whether numbers are sequential or random.
2. Propose two distinct approaches: First, suggest a boolean array for dense ranges offering O(1) lookup but high memory overhead. Second, propose a hash set or sorted linked list for sparse data, highlighting O(1) average time complexity for insertions/deletions.
3. Discuss optimization strategies: Explain how to handle the 'release' operation efficiently by maintaining a pool of available numbers or using a doubly linked list to avoid traversal.
4. Compare trade-offs explicitly: Analyze space complexity (O(N) for arrays vs O(K) for lists where K is active numbers) and time complexity under load.
5. Write clean code: Implement the chosen solution with clear variable names, handling edge cases like empty directories or full capacity gracefully.
Key Points to Cover
- Explicitly defining the trade-off between space complexity and time complexity based on data sparsity
- Demonstrating knowledge of O(1) operations using Hash Sets for fast lookups and updates
- Proposing a specific mechanism to retrieve any available number efficiently without scanning
- Handling edge cases such as a full directory or invalid input gracefully
- Aligning the solution with high-performance standards typical of fintech companies like Stripe
Sample Answer
To design this directory, I first need to understand if the number range is fixed and dense, like local area codes, or highly sparse across global prefixes. If we assume a standard scenario where numbers are sparse, a si…
Common Mistakes to Avoid
- Suggesting a brute-force linear scan to find an available number, resulting in O(N) time complexity
- Ignoring memory constraints by proposing a massive boolean array for sparse phone number ranges
- Failing to clarify whether the system needs to support concurrent access or just single-threaded operations
- Implementing a singly linked list which makes removing released numbers inefficient compared to a doubly linked list
Sound confident on this question in 5 minutes
Answer once and get a 30-second AI critique of your structure, content, and delivery. First attempt is free — no signup needed.