[leetcode] 398. Random Pick Index 解题报告

题目链接：https://leetcode.com/problems/random-pick-index/

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.

Note:

The array size can be very large. Solution that uses too much extra space will not pass the judge.

Example:

int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);

// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);

// pick(1) should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(1);

思路：水池取样算法，水池大小为１，因此对k个target只要用概率1/k来决定是否替换水池中的数即可．

代码如下：

class Solution {
public:
Solution(vector<int> nums){
copy = nums;
}

int pick(int target) {
int ans;
for(int i = 0, cnt = 1; i < copy.size(); i++)
if(copy[i]==target && rand()%(cnt++)==0) ans = i;
return ans;
}
private:
vector<int> copy;
};

/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(nums);
* int param_1 = obj.pick(target);
*/