Apple·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Apple SWE interview with a system design coding problem that was more nuanced than it looked on the surface. The question was about an in-memory hit counter and it required thinking carefully about data structure tradeoffs and space constraints.

Questions Asked (1)

Design an in-memory hit counter with two operations: one to record a hit at a given timestamp, and one to return the total number of hits in the last 300 seconds. Timestamps are integer seconds and arrive in non-decreasing order. Aim for O(1) amortized time per operation and O(300) space, and explain how you'd expire old data without storing every hit individually. Write the API and unit tests.

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

I went straight to a circular buffer of size 300, keyed by timestamp mod 300, storing a count per second bucket.

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AI HintsAI Generated

Suggested Approach

Clarify the API and constraints, then propose a ring buffer of 300 buckets indexed by timestamp modulo 300, where each bucket stores the count of hits for that second. On each hit, update the bucket for the current timestamp, resetting it if it's stale (older than 300 seconds), and maintain a running total; on query, return the total. Explain that this achieves O(1) amortized time and O(300) space by aggregating hits per second and expiring old data via the circular index.

Pro tip: Emphasize that the non-decreasing timestamp guarantee simplifies expiration: you only need to check the bucket you're about to overwrite, not scan all buckets. Also, mention that you'd handle out-of-order timestamps gracefully by ignoring or logging them, since the problem states they won't occur.

1. Clarify requirements and constraints

Confirm the API signatures, the window size (300 seconds), and that timestamps are non-decreasing integers. Discuss edge cases like multiple hits in the same second and queries at the same timestamp.

2. Design the data structure

Propose a circular buffer (array) of size 300, where each slot stores the count of hits for a specific second. Use the timestamp modulo 300 as the index, and keep a running total of hits in the window.

3. Define the algorithms

For recordHit(timestamp): compute index = timestamp % 300; if the bucket's stored timestamp is not equal to timestamp, reset the bucket to 0 and subtract its old count from the total; then increment the bucket and total. For getHits(): return the total.

4. Analyze complexity and trade-offs

Explain that both operations are O(1) amortized (each hit does constant work) and space is O(300). Discuss why storing individual hits would be O(n) space and slower, and how the circular buffer aggregates data.

5. Write unit tests

Outline tests for: no hits, single hit, multiple hits in same second, hits spanning the 300-second boundary, and queries at various times. Include a test for the non-decreasing timestamp assumption.

Key Points to Mention

Circular buffer of size 300 indexed by timestamp modulo 300
Each bucket stores the count of hits for that specific second
Maintain a running total to avoid summing all buckets on query
Expiration: when overwriting a bucket, check if its timestamp is stale (older than 300 seconds) and subtract its count from the total
O(1) amortized time per operation, O(300) space
Handling multiple hits in the same second by incrementing the bucket count
Unit tests covering boundary conditions and the non-decreasing timestamp guarantee

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