DesignPhoneDirectory.java

package medium;

import java.util.HashSet;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Set;

public class DesignPhoneDirectory {

    private class PhoneDirectory {
        private Queue<Integer> phoneDir;
        private Set<Integer> used;
        /**
         * Initialize your data structure here
         * 
         * @param maxNumbers
         *            - The maximum numbers that can be stored in the phone directory.
         */
        public PhoneDirectory(int maxNumbers) {
            phoneDir = new LinkedList();
            int number = 0;
            while (maxNumbers-- > 0){
                phoneDir.add(number++);
            }
            used = new HashSet();
        }

        /**
         * Provide a number which is not assigned to anyone.
         * 
         * @return - Return an available number. Return -1 if none is available.
         */
        public int get() {
            if (phoneDir.peek() == null) return -1;
            int newNumber = phoneDir.poll();
            used.add(newNumber);
            return newNumber;
        }

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

        /** Recycle or release a number. */
        public void release(int number) {
            if (used.remove(number)) {
                phoneDir.add(number);
            }
        }
    }

}