[leetcode] 384. Shuffle an Array

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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();

这道题是实现洗牌算法，题目难度为Medium。

问题的本质是从n!种（n为数组长度）排列组合中等概率选取一种，可以从每个位置着手，逐个选出该位置数字来得到一种组合。从最后一个数字开始，随机取数组中任意数字和它交换，该位置数字即视为选定了；然后确定倒数第二个数字，随机取前n-1个数字中任意数字和它交换，该位置数字也选定了；重复以上操作直至第一个数字即完成了数组的洗牌。具体代码：

class Solution {    vector<int> orgi, data;public:    Solution(vector<int> nums) : data(nums), orgi(nums) {}        /** Resets the array to its original configuration and return it. */    vector<int> reset() {        return orgi;    }        /** Returns a random shuffling of the array. */    vector<int> shuffle() {        for(int i=data.size()-1; i>=0; --i)            swap(data[i], data[rand()%(i+1)]);        return data;    }};/** * 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(); */

感兴趣的同学可以看一下wiki上关于洗牌算法的页面：https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle

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