Email Notification Scheduler

Design and implement an Email Notification Scheduler. The system receives scheduling requests, each with: userId notificationType sendAt (scheduled time) payload Implement the scheduling logic so that as time advances you can output which emails should be sent now. Common follow-ups (as specified by the interviewer) include: de-duplication/merging for the same user + type within a time window per-user rate limiting ordering when multiple tasks share the same sendAt out-of-order arrivals, cancellation/updates (if required) Requirement: justify your data structures and handle corner/edge cases. Note: Exact I/O and rules were not provided in the post; this is a reconstructed version suitable for practice.