The 60-second answer
Use a hash map keyed by message/entity with timestamps, optionally paired with a queue for cleanup. Target O(1) average lookup/update; cleanup cost amortized when a queue is used.
Build the answer in this order
1
Clarify constraints
Use a hash map keyed by message/entity with timestamps, optionally paired with a queue for cleanup.
2
Choose the approach
Target O(1) average lookup/update; cleanup cost amortized when a queue is used.
3
Prove complexity
Call out edge cases such as out-of-order timestamps, duplicate events, expiration boundaries, and unbounded memory.
4
Test edge cases
Explain the invariant before coding, then dry-run a small case and state time/space complexity.
A useful interview mental model
This is the shape of a strong answer—not a script to memorize.
01Clarify
02Approach
03Implement
04Test
05Complexity
Senior-level signal
- Ask whether event time is monotonic; that determines whether queue-based expiry is valid.
- Discuss memory reclamation and concurrent callers when turning the interview solution into a service.
What the interviewer is really testing
Problem decomposition, correctness, data-structure choice, complexity reasoning, and clean implementation under pressure.
Likely follow-up questions
Can you improve the time or space complexity?
Which edge case is most likely to break this solution?
How would you test this under interview time pressure?
Common weak-answer patterns
- Starting to code before constraints are clear.
- Giving complexity without explaining why it is correct.
- Skipping adversarial and boundary cases.