#957 · #2622 · Cache With Time Limit(有时间限制的缓存)

Implement a TimeLimitedCache class: set(key, value, duration) stores an entry and returns whether the key already existed and was unexpired; get(key) returns the value if unexpired, otherwise -1; count() returns the number of unexpired entries. Constraints: at most 2 * 10^5 calls.