Shuffle an Array (reservoir sampling)

Randomness

Reservoir sampling algorithm ensures that each sample is pumped equal probability

Where k samples selected from the set S of n items comprising, where n is a number of large or unknown, especially for not all the n items are stored to the main memory: a scene using

Knuth shuffle algorithm

Pick up the first itime cards, which were randomly selected from the front of the cardj，或从它后面的牌随机选出j交换即可

 1 class Solution {
 2 public:
 3     Solution(vector<int>& nums) {
 4         v = nums;
 5     }
 6     
 7     /** Resets the array to its original configuration and return it. */
 8     vector<int> reset() {
 9         return v;
10     }
11     
12     /** Returns a random shuffling of the array. */
13     vector<int> shuffle() {
14         vector<int> res = v;
15         for (int i = 0; i < res.size(); ++i) {
16             int t = i + rand() % (res.size() - i);
17             swap(res[i], res[t]);
18         }
19         return res;
20     }
21     vector<int> v;
22 };
23 
24 /**
25  * Your Solution object will be instantiated and called as such:
26  * Solution* obj = new Solution(nums);
27  * vector<int> param_1 = obj->reset();
28  * vector<int> param_2 = obj->shuffle();
29  */