379-design-phone-directory

Question

https://leetcode.com/problems/design-phone-directory/description/

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);

Thought Process

1. Queue and Boolean array
   1. Time complexity O(1)
   2. Space complexity O(n)

Solution

class PhoneDirectory {
    Queue<Integer> queue;
    boolean[] used;

    /** 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<>();
        for (int i = 0; i < maxNumbers; i++) {
            queue.offer(i);
        }
        used = new boolean[maxNumbers];
    }

    /** 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()) return -1;
        used[queue.peek()] = true;
        return queue.poll();
    }

    /** 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]) {
            used[number] = false;
            queue.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);
 */

Additional