Question

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

class PhoneDirectory {
    LinkedHashSet<Integer> availableNumbers;
    int availableCount;

    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        availableNumbers = new LinkedHashSet<Integer>(maxNumbers, 1.0f);
        for(int i = 0; i < maxNumbers; i++) availableNumbers.add(i);
        availableCount = 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(availableCount == 0) return -1;
        else {
            availableCount--;
            int retVal = availableNumbers.iterator().next();
            availableNumbers.remove(retVal);
            return retVal;
        }
    }

    /** Check if a number is available or not. */
    public boolean check(int number) {
        return availableNumbers.contains(number);
    }

    /** Recycle or release a number. */
    public void release(int number) {
        if(!availableNumbers.contains(number)) {
            availableNumbers.add(number);
            availableCount++;
        }
    }
}

/**
 * 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);
 */