# Question

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.
Example:

// 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.
directory.get();

// Assume it returns 1.
directory.get();

// The number 2 is available, so return true.
directory.check(2);

// It returns 2, the only number that is left.
directory.get();

// The number 2 is no longer available, so return false.
directory.check(2);

// Release number 2 back to the pool.
directory.release(2);

// Number 2 is available again, return true.
directory.check(2);


# Solution

class PhoneDirectory {
int availableCount;

/** Initialize your data structure here
@param maxNumbers - The maximum numbers that can be stored in the phone directory. */
public PhoneDirectory(int maxNumbers) {
for(int i = 0; i < maxNumbers; i++) availableNumbers.add(i);
availableCount = maxNumbers;
}

/** Provide a number which is not assigned to anyone.
@return - Return an available number. Return -1 if none is available. */
public int get() {
if(availableCount == 0) return -1;
else {
availableCount--;
int retVal = availableNumbers.iterator().next();
availableNumbers.remove(retVal);
return retVal;
}
}

/** Check if a number is available or not. */
public boolean check(int number) {
return availableNumbers.contains(number);
}

/** Recycle or release a number. */
public void release(int number) {
if(!availableNumbers.contains(number)) {