public class PhoneDirectory {

     Set<Integer> used = new HashSet<>();

     Queue<Integer> available = new LinkedList<>();

     int max;

     public PhoneDirectory(int maxNumbers) {

         max = maxNumbers;

         for (int i = ; i < maxNumbers; i++) {

             available.offer(i);

         }

     }

     public int get() {

         Integer ret = available.poll();

         if (ret == null) {

             return -;

         }

         used.add(ret);

         return ret;

     }

     public boolean check(int number) {

         if (number >= max || number < ) {

             return false;

         }

         return !used.contains(number);

     }

     public void release(int number) {

         if (used.remove(number)) {

             available.offer(number);

         }

     }

 }