Shuffle an Array (水塘抽样)

随机性问题

水塘抽样算法可保证每个样本被抽到的概率相等

使用场景：从包含n个项目的集合S中选取k个样本，其中n为一很大或未知的数量，尤其适用于不能把所有n个项目都存放到主内存的情况

Knuth洗牌算法

拿起第i张牌时，只从它前面的牌随机选出j，或从它后面的牌随机选出j交换即可

class Solution {
public:
    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;
    }
    vector<int> v;
};

/**
 * 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();
 */