#166 · #379 · Design Phone Directory(电话目录)

Design a phone directory that assigns numbers in the range [0, maxNumbers - 1]. Implement get() returning an available number (or -1 if none), check(number) returning whether the number is available, and release(number) returning a number to the pool. Constraints: 1 <= maxNumbers <= 10^4; at most 10^4 calls.