LeetCode – Shuffle an Array (Java)

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Shuffle a set of numbers without duplicates.

Java Solution

How we make sure each the probability of each element get shuffled is very similar to the streaming random problem.

The algorithm is straightforward to understand, but the question is why it works. To have a working shuffle algorithm, every element in the array results in each position should be equal. 

class Solution {
    int[] original = null;
    int[] shuffle = null;
    Random rand = null;
 
    public Solution(int[] nums) {
        original = nums;
        shuffle = Arrays.copyOf(nums, nums.length);
        rand = new Random();
    }
 
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        shuffle = Arrays.copyOf(original, original.length);
        return shuffle;
    }
 
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        for(int i=0; i<shuffle.length; i++){
            int x = rand.nextInt(shuffle.length-i);
            int idx = x+i;
 
            int tmp = shuffle[idx];
            shuffle[idx] = shuffle[i];
            shuffle[i] = tmp;
        }
 
        return shuffle;
    }
}

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