379. Design Phone Directory

Problem:

Design a Phone Directory which supports the following operations:

1. get: Provide a number which is not assigned to anyone.
2. check: Check if a number is available or not.
3. 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);

Analysis:
一开始题意理解错了。get是提供没有用过的。
Use queue and set, set stores the numbers that have been used. The number to be check might less than 0 or greater than max number. So we need to check this as well.

Solution:

 class PhoneDirectory {
    Set<Integer> set;
    Queue<Integer> queue;
    int max;
    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        queue = new LinkedList<>();
        set = new HashSet<>();
        max = maxNumbers;
        for (int i = 0; i < maxNumbers; i++) {
            queue.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 (!queue.isEmpty()) {
            set.add(queue.peek());
            return queue.poll();
        } else {
            return -1;
        }
    }
    
    /** Check if a number is available or not. */
    public boolean check(int number) {
        if (number > max  || number < 0)
            return false;
        return !set.contains(number);
    }
    
    /** Recycle or release a number. */
    public void release(int number) {
        if (set.remove(number)) {
            queue.offer(number);
        }
    }
}