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Apple·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Apple SWE interview with a classic data structures problem. Pretty standard coding round, one question, focused on whether you actually understand stack mechanics well enough to fake a queue with them.

Questions Asked (1)

Q1

Implement a FIFO queue using exactly two LIFO stacks, supporting push, pop, peek, and empty operations. Bonus: can you get amortized O(1) per operation?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The two-stack trick is one of those things where you either know it or you sit there staring at the screen for an uncomfortable amount of time.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and constraints, then explain the two-stack approach where one stack handles enqueue and the other handles dequeue. Walk through the amortized O(1) analysis and discuss trade-offs.

Pro tip: Emphasize that while individual operations can be O(n), the amortized cost is O(1) because each element is moved at most once between stacks. This demonstrates understanding of amortized analysis, which is highly valued at Apple.

1. Clarify Requirements

Confirm the operations needed (push, pop, peek, empty) and discuss any constraints like thread safety or memory limits.

2. Design the Two-Stack Approach

Use one stack (inbox) for enqueue and another (outbox) for dequeue. When outbox is empty, transfer all elements from inbox to outbox.

3. Implement Operations

For push, add to inbox. For pop/peek, if outbox is empty, move all from inbox to outbox, then pop/peek from outbox. For empty, check both stacks.

4. Analyze Complexity

Explain that each element is moved at most once from inbox to outbox, so amortized time per operation is O(1). Space is O(n).

5. Discuss Trade-offs

Mention alternatives like using a linked list or circular buffer, and compare performance characteristics.

Key Points to Mention

  • Two stacks: inbox for enqueue, outbox for dequeue
  • Transfer elements only when outbox is empty
  • Amortized O(1) per operation due to each element moved at most once
  • Worst-case O(n) for a single pop/peek when transfer occurs
  • Space complexity O(n)
  • Thread safety considerations if needed

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.