Leetcode: 384.打乱数组

题目描述

打乱一个没有重复元素的数组。

示例:

// 以数字集合 1, 2 和 3 初始化数组。
int[] nums = {1,2,3};
Solution solution = new Solution(nums);

// 打乱数组 [1,2,3] 并返回结果。任何 [1,2,3]的排列返回的概率应该相同。
solution.shuffle();

// 重设数组到它的初始状态[1,2,3]。
solution.reset();

// 随机返回数组[1,2,3]打乱后的结果。
solution.shuffle();

思路

思路1：复制数组+移除

复制一个新数组，然后随机置入数组，将置入数组的数移除

思路2：Fisher–Yates shuffle

遍历数组，随机化得到一个从当前位置到最后位置的数进行交换，该方法产生n!种可能，但是如果采用随机产生两个随机数则是错的，其产生的可能性为$n^n$,简单证明为n!可能不能被$n^n$整除

代码

代码1: 复制数组+移除

class Solution {
    private int[] nums;
    private int[] origins;
    
    private Random rand = new Random();
    
    public Solution(int[] nums) {
        this.nums = nums;
        origins = this.nums.clone();
    }
    
    /** Resets the array to its original configuration and return it. */
    public int[] reset() {
        this.nums = this.origins;
        this.origins = this.origins.clone();
        return this.nums;
    }
    
    /** Returns a random shuffling of the array. */
    public int[] shuffle() {
        List copyList = new ArrayList<>();
        for(int num: this.nums){
            copyList.add(num);
        }
        for(int i = 0; i < this.nums.length; i++){
            int removeIndex = rand.nextInt(copyList.size());
            this.nums[i] = copyList.get(removeIndex);
            copyList.remove(removeIndex);
        }
        return this.nums;
    }
}

代码2

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