I started by just summing end minus start for each session and almost submitted that before catching the overlap issue.
First, clarify the input format and edge cases (e.g., malformed sessions, overlapping intervals, multiple sessions per user). Then, for each user, collect valid intervals, sort them by start time, and merge overlapping intervals to compute the union length. Finally, sum the merged durations per user and return the result.
Pro tip: Mention that you would handle malformed sessions early by filtering them out, and discuss how to scale the solution for large streams (e.g., using a heap or streaming merge). Also, consider if sessions can be open-ended (no end timestamp) and how to handle that.
Ask about input format (e.g., list of events, each with start/end timestamps and user ID), whether sessions can be open-ended, and how to handle malformed sessions. Confirm that billable seconds are the union of intervals per user.
Discard malformed sessions where end < start. Group valid sessions by user ID, storing intervals as (start, end) pairs.
For each user, sort intervals by start time. Iterate through sorted intervals, merging overlapping ones by updating the end time to the maximum of current end and next end.
Sum the lengths of the merged intervals for each user. Return a mapping of user ID to total billable seconds.
Discuss time complexity: O(N log N) per user due to sorting, or O(N log N) overall if sorting all intervals globally. Mention potential optimizations for streaming data, such as using a balanced tree or heap.
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