384. Shuffle an Array

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();

My Solutions:

Have an int[] arr to record the original array;

for shuffle(), swap the index i and index j, where j = random.nextInt(len - i) + i (j是i之后的任意index)

(Resorvior sampling: the probability to choose an element out of n elements is 1/n)

class Solution {
    
    private int[] original;
    Random r = new Random();

    public Solution(int[] nums) {
        this.original = nums;
    }
    
    /** 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() {
        int[] arr = original.clone();
        int len = arr.length;
        
        for (int i = 0; i < len; i++) {
            int j = r.nextInt(len - i) + i;
            swap(arr, i, j);
        }
        return arr;
    }
    
    private void swap(int[] arr, int i, int j) {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
}