379_Design Phone Directory.java

class PhoneDirectory {
    boolean[] used;
    Stack<Integer> stack;

    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        used = new boolean[maxNumbers];
        stack = new Stack<>();
        for (int i = 0; i <maxNumbers; ++i) {
            stack.push(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 (stack.isEmpty()) {
            return -1;
        }
        int num = stack.pop();
        used[num] = true;
        return num;
    }
    
    /** Check if a number is available or not. */
    public boolean check(int number) {
        return !used[number];
    }
    
    /** Recycle or release a number. */
    public void release(int number) {
        if (used[number]) {
            stack.push(number);
            used[number] = false;
        }        
    }
}

/**
 * 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);
 */