Design a Phone Directory which supports the following operations:
get: Provide a number which is not assigned to anyone.
check: Check if a number is available or not.
release: Recycle or release a number.
// Init a phone directory containing a total of 3 numbers: 0, 1, and 2.
PhoneDirectory directory = new PhoneDirectory(3);
// It can return any available phone number. Here we assume it returns 0.
// Assume it returns 1.
// The number 2 is available, so return true.
// It returns 2, the only number that is left.
// The number 2 is no longer available, so return false.
// Release number 2 back to the pool.
// Number 2 is available again, return true.