ShuffleAnArray.java

package com.fishercoder.solutions;
/**384. Shuffle an Array
Shuffle a set of numbers without duplicates.

Example:

// Init an array with set 1, 2, and 3.
int[] nums = {1,2,3};
Solution solution = new Solution(nums);

// Shuffle the array [1,2,3] and return its result. Any permutation of [1,2,3] must equally likely to be returned.
solution.shuffle();

// Resets the array back to its original configuration [1,2,3].
solution.reset();

// Returns the random shuffling of array [1,2,3].
solution.shuffle();*/
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Random;

public class ShuffleAnArray {
    
    public static void main(String...strings){
        int[] nums = new int[]{1,2,3};
        Solution_for_this_question test = new Solution_for_this_question(nums);
    }

}
class Solution_for_this_question {
    //Note: the problem states that this is a set without duplicates which makes building all combinations easier
    
    private List<List<Integer>> combinations;
    private int[] original;
    private Random random;
    
    public Solution_for_this_question(int[] nums) {
        original = nums;
        random = new Random();
        combinations = buildAllComb(nums);
    }
    
    //insert next value into all possible positions, I wrote this method myself, of course it could be simplified to not use a queue
    //but it just naturally came into my mind that I used a queue
    private List<List<Integer>> buildAllComb(int[] nums) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(nums == null || nums.length == 0) return result;
        
        List<Integer> list = new ArrayList<Integer>();
        list.add(nums[0]);
        Queue<List<Integer>> q = new LinkedList();
        q.offer(list);
        for(int i = 1; i < nums.length; i++){
            int qSize = q.size();
            for(int k = 0; k < qSize; k++){
                List<Integer> currList = q.poll();
                for(int j = 0; j <= currList.size(); j++){
                    List<Integer> newL = new ArrayList<Integer>(currList);
                    newL.add(j, nums[i]);
                    q.offer(newL);
                }
            }
        }
        while(!q.isEmpty()){
            result.add(q.poll());
        }
        return result;
    }

    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        return original;
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        if(original == null || original.length == 0) return original;
        int randomIndex = random.nextInt(combinations.size());
        List<Integer> list = combinations.get(randomIndex);
        int[] result = new int[list.size()];
        for(int i = 0; i < list.size(); i++){
            result[i] = list.get(i);
        }
        return result;
    }
}