The prompt they gave was maybe three sentences.
Clarify the problem constraints and edge cases, then propose a two-pointer merge approach that leverages the sorted order of both streams to find the minimum absolute difference in O(n+m) time per user. Discuss how to handle users present in only one stream and the overall time complexity when processing multiple users.
Pro tip: Mention that since both streams are sorted, a two-pointer technique is optimal and avoids unnecessary comparisons; also note that if the streams were unsorted, sorting would add O(n log n) overhead, so the sorted property is key.
Ask about input format, whether timestamps are integers or floats, if streams can be empty, and if users may have multiple orders/ads. Confirm that we need the minimum difference per user, not globally.
Use two pointers, one for each stream, starting at the beginning. Since both are sorted, advance the pointer with the smaller timestamp and compute the absolute difference at each step, updating the minimum.
Group events by user ID (e.g., using hash maps) or process each user's streams separately. Only consider users that appear in both streams; skip others.
Explain that the two-pointer approach runs in O(n + m) time per user and O(1) extra space. If processing many users, total time is O(N + M) where N and M are total events across all users.
Walk through a simple example, then test edge cases: empty streams, single event, identical timestamps, and users with no overlap. Verify the algorithm returns the correct minimum difference.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.