Find returning users from access logs
Company: Amazon
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Onsite
Given an unordered log of web access events, identify which users are returning (i.e., they have visited on at least two different calendar days). Each event is a tuple (user_id, timestamp_iso
8601). Output the set of user_ids that meet the criterion. Address:
1) parsing and time-zone normalization;
2) handling large logs that don't fit in memory;
3) algorithmic complexity;
4) test cases including edge cases near midnight and duplicate events.
Quick Answer: This question evaluates skills in time-series data processing, timestamp parsing and time-zone normalization, scalable stream or batch handling for logs that exceed memory, algorithmic complexity analysis, and test-case design for edge conditions such as midnight boundaries and duplicate events.
