[LeetCode] Design Phone Directory

Problem

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

Set<Integer> used = new HashSet<Integer>();
    Queue<Integer> available = new LinkedList<Integer>();
    int max;
    public PhoneDirectory(int maxNumbers) {
        max = maxNumbers;
        for (int i = 0; i < maxNumbers; i++) {
            available.offer(i);
        }
    }
    
    public int get() {
        Integer ret = available.poll(); //Use Integer in case ret is null
        if (ret == null) {
            return -1; //Handle null exception
        }
        used.add(ret);
        return ret;
    }
    
    public boolean check(int number) { //Use HashSet only
        if (number >= max || number < 0) {
            return false;
        }
        return !used.contains(number); 
    }
    
    public void release(int number) { //from used to available
        if (used.remove(number)) {
            available.offer(number);
        }
    }