Multi-Level In-Memory Database with TTL and Time Travel

Problem: In-Memory Database with TTL and Time Travel Implement a simplified in-memory database storing string values by (key, field). It must support basic writes, reads, deletes, scans, TTL expiration, and historical point-in-time queries. Input The first line contains an integer Q. Each of the next Q lines is one operation: SET t key field value: set key.field = value at time t with no expiration. SET_TTL t key field value ttl: set the value at time t, expiring at t + ttl; valid interval is [t, t + ttl). GET t key field: return the value valid at time t. DELETE t key field: delete the field at time t if it is currently valid. SCAN t key: return all valid fields under key at time t, sorted by field name, formatted as field=value; output null if none exist. GET_AT t key field at_time: at current time t, query the value at historical time at_time. Timestamps are non-decreasing in input order. Output For every GET, SCAN, and GET_AT, print one line. Print null if no value exists. Constraints 1 <= Q <= 2 * 10^5 key, field, and value length at most 50 1 <= ttl timestamps are non-negative integers Example Input 7 SET 1 user name alice GET 2 user name DELETE 3 user name GET 4 user name SET_TTL 5 user name bob 3 GET 7 user name GET 8 user name Output alice null bob null