Design Phone Directory

Description

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);

Hint

Train of Thought

all consideration is time complexity and how to get 1st element use a queue to store available numbers and a set to store used number is good

Code

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();
        if (ret == null) {
            return -1;
        }
        used.add(ret);
        return ret;
    }

    public boolean check(int number) {
        if (number >= max || number < 0) {
            return false;
        }
        return !used.contains(number);
    }

    public void release(int number) {
        if (used.remove(number)) {
            available.offer(number);
        }
    }

Complexity