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();

这个用到了洗牌算法：
1）将第1个元素与n个元素中的任意一个交换(包括自己)。这时结果中第1个元素已经确定。
2）将第2个元素与n-1个元素中作任意一个交换(包括自己)。
3）重复上面步骤，直到剩1个元素为止。
这样就可以返回随机排列的数组了。

public class Solution {
    private int[] original;
    private Random rng = new Random();
    private int[] copy;
    private int num;
    public Solution(int[] nums) {
        original = nums;
        copy=original.clone();
        num=original.length;
    }
    
    /** 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 n = num;
        while(n>1) {
            n--;
            int k = rng.nextInt(n + 1);
            int value = copy[k];
            copy[k] = copy[n];
            copy[n] = value;
        }
        return copy;
    }
}