384. Shuffle an Array

class Solution {
    private int[] nums;
    private final int[] clonedNums;

    public Solution(int[] nums) {
        this.nums = nums;
        this.clonedNums = nums.clone();
    }

    public int[] reset() {
        nums = clonedNums.clone();
        return nums;
    }

    public int[] shuffle() {
        for (int i = 0; i < nums.length; i++) {
            int randomIndex = ThreadLocalRandom.current().nextInt(i, nums.length);
            swap(nums, i, randomIndex);
        }
        return nums;
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}

每次从剩余的元素中选择一个，保证了排列数为 n! 个。

References

384. Shuffle an Array