class Solution {
    private int[] nums;
    private Random random;

    public Solution(int[] nums) {
        this.nums = nums;
        random = new Random();
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        return nums;
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        if (nums == null)
                return null;
            int[] a = nums.clone();//clone一个新的数组
            for (int j = 1; j < a.length; j++) {
                int i = random.nextInt(j + 1);		//生成一个随机的int值，该值介于[0, j+1)的区间，也就是0到 j + 1 之间的随机int值，包含0而不包含j + 1。
                swap(a, i, j);
            }
            return a;
    }

    private void swap(int[] a, int i, int j) {
        if (i != j) {
            int temp = a[j];
            a[j] = a[i];
            a[i] = temp;
        }
    }

}