384. Shuffle an Array

Middle
一开始很naive地觉得这道题就是考backtracking, 找到所有的permutations然后返回任意一个.然而闪亮亮的TLE，说明这道题不是想考你这个. 注意一下`r.nextInt(i + 1)'这个方法返回的是[0, i]中的任意一个int. 要返回[min, max]中的任意一个int, 可以用以下方法：

Random r = new Random();
return r.nextInt((max - min) + 1) + min;

Fisher–Yates shuffle Algorithm

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class Solution {
    int[] orig;
    public Solution(int[] nums) {
        this.orig = nums;
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        return orig;    
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        int[] arr = orig.clone();
        Random r = new Random();
        for (int i = arr.length - 1; i > 0; i--){
            int j = r.nextInt(i + 1);
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        } 
        return arr;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int[] param_1 = obj.reset();
 * int[] param_2 = obj.shuffle();
 */

还有一个方向相反思路相同的解法：

class Solution {
    int[] orig;
    public Solution(int[] nums) {
        this.orig = nums;
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        return orig;    
    }
    
    //r.nextInt((max - min) + 1) + min;
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        int[] arr = orig.clone();
        Random rand = new Random();
        for (int i = 0; i < arr.length - 1; i++){
            //pick randrom j that 0 <= j <= i
            int j = rand.nextInt(arr.length - 1 - i + 1) + i;
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        } 
        return arr;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int[] param_1 = obj.reset();
 * int[] param_2 = obj.shuffle();
 */

高能重点来了，面经里看到别人被问：
a. When do shuffling, how to make sure each num is picked equally? Prove it.

b. How many permutation can you get? n!

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