My code:

public class PhoneDirectory {
    boolean[] map;
    Queue<Integer> q;
    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        map = new boolean[maxNumbers];
        q = new LinkedList<Integer>();
        for (int i = 0; i < maxNumbers; i++) {
            q.offer(i);
        }
    }
    
    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    public int get() {
        if (q.isEmpty()) {
            return -1;
        }
        else {
            int ret = q.poll();
            map[ret] = true;
            return ret;
        }
    }
    
    /** Check if a number is available or not. */
    public boolean check(int number) {
        return !map[number];
    }
    
    /** Recycle or release a number. */
    public void release(int number) {
        if (map[number]) {
            map[number] = false;
            q.offer(number);
        }
    }
}

/**
 * Your PhoneDirectory object will be instantiated and called as such:
 * PhoneDirectory obj = new PhoneDirectory(maxNumbers);
 * int param_1 = obj.get();
 * boolean param_2 = obj.check(number);
 * obj.release(number);
 */

不是很难，直接拿 Queue做了。

Anyway, Good luck, Richardo! -- 09/11/2016