leetcode 384. Shuffle an Array 数组随机洗牌-蒲公英云

leetcode 384. Shuffle an Array 数组随机洗牌

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();

网上看到的代码，就这么吧！

代码如下：

public class Solution {
    private int[] nums = null;
    private Random random = null;
    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 = (int[])nums.clone();
        for(int i = 1; i < nums.length; i++){
            int j = random.nextInt(i + 1);
            swap(a, i, j);
        }
        return a;
    }
    private void swap(int[] a, int i, int j){
        int temp = a[i];
        a[i] = a[j];
        a[j] = temp;
    }
}
/**
 * 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();
 */

下面是C++的做法

这道题让我们给数组洗牌，也就是随机打乱顺序，那么由于之前那道题leetcode 382. Linked List Random Node 等概率随机获取结点 + 蓄水池算法我们接触到了水塘抽样的思想，这道题实际上这道题也是用类似的思路，我们遍历数组每个位置，每次都随机生成一个坐标位置，然后交换当前遍历位置和随机生成的坐标位置的数字，这样如果数组有n个数字，那么我们也随机交换了n组位置，从而达到了洗牌的目的，这里需要注意的是i + rand() % (res.size() - i)不能写成rand() % res.size()，虽然也能通过OJ，但是根据这个帖子的最后部分的概率图表，前面那种写法不是真正的随机分布，应该使用Knuth shuffle算法，感谢热心网友们的留言，

代码如下：

#include <iostream>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <queue>
#include <stack>
#include <string>
#include <climits>
#include <algorithm>
#include <sstream>
#include <functional>
#include <bitset>
#include <numeric>
#include <cmath>
#include <regex>
using namespace std;
class Solution 
{
public:
    vector<int> v;
    Solution(vector<int> nums) 
    {
        v = nums;
    }
    /** Resets the array to its original configuration and return it. */
    vector<int> reset()
    {
        return v;
    }
    /** Returns a random shuffling of the array. */
    vector<int> shuffle() 
    {
        vector<int> res = v;
        for (int i = 0; i < res.size(); i++)
        {
            int t = i + rand() % (res.size() - i);
            swap(res[i], res[t]);
        }
        return res;
    }
};
/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(nums);
* vector<int> param_1 = obj.reset();
* vector<int> param_2 = obj.shuffle();
*/