LeetCode-Design Phone Directory

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:

 

We record the last used number and also maintain a hashset to record all released number. Everytime get() called, we first get a released number, if not, then use an unused number.

 

Solution:

public class PhoneDirectory {
    HashSet<Integer> numReleased;
    int maxNum, nextUnuse;


    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        numReleased = new HashSet<Integer>();
        maxNum = maxNumbers;
        nextUnuse = 0;
        
    }
    
    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    public int get() {
        int num = -1;
        if (!numReleased.isEmpty()) {
            for (Integer i : numReleased) {
                num = i;
                break;
            }
            numReleased.remove(num);
        } else if (nextUnuse < maxNum) {
            num = nextUnuse++;
        }
        return num;
    }
    
    /** Check if a number is available or not. */
    public boolean check(int number) {
        if (number >= maxNum) return false;

        if (number >= nextUnuse || numReleased.contains(number)) return true;

        return false;
    }
    
    /** Recycle or release a number. */
    public void release(int number) {
        if (number >= nextUnuse)  return;
        numReleased.add(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);
 */