Leetcode: Design Phone Directory

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);

my HashSet+ ArrayList, 删除的时候把要删的index与末尾对调。get()其实不需要random, 因为anyone is ok

public class PhoneDirectory {
    ArrayList<Integer> arr;
    HashSet<Integer> set;

    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        arr = new ArrayList<Integer>();
        set = new HashSet<Integer>();
        for (int i=0; i<maxNumbers; i++) {
            arr.add(i);
            set.add(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 (arr.size() == 0) return -1;
        Random random = new Random();
        int index = random.nextInt(arr.size());
        int temp = arr.get(index);
        arr.set(index, arr.get(arr.size()-1));
        arr.set(arr.size()-1, temp);
        arr.remove(arr.size()-1);
        set.remove(temp);
        return temp;
    }
    
    /** Check if a number is available or not. */
    public boolean check(int number) {
        return set.contains(number);
    }
    
    /** Recycle or release a number. */
    public void release(int number) {
        if (!set.contains(number)) {
            arr.add(number);
            set.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);
 */

HashSet+ Queue网上vote最高的solution, 

public class PhoneDirectory {

    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);
        }
    }
}

/**
 * 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);
 */