Cache With Time Limit

Design a time-limited key-value store with set(key, value, duration) that returns true if it overwrote an unexpired key (false otherwise), get(key) returning the value or -1 if expired, and count() returning the number of unexpired keys. The core challenge is efficiently tracking per-key expiry times and handling expirations (e.g., via timestamped hashmap with lazy cleanup or a min-heap) so get/count operations reflect only unexpired entries.