398. Random Pick Index

问题描述：

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);

解题思路：

这道题要求我们随机返回一个给定目标数字的下标。

并且数组可能十分之大而我们不能用过多的额外空间。

这个时候可以用水塘抽样算法来解决

当我们遇见一个数字等于target时，total++。

同时判断是否取当前数字。

我们通过rand()随机生成一个数字，使之对total求余，若余数为0，则替换，否则不替换

代码：

class Solution {
public:
    Solution(vector<int> nums) {
        v = nums;
    }
    
    int pick(int target) {
        int ret = 0;
        int total = 0;
        for(int i = 0; i < v.size(); i++){
            if(v[i] == target){
                total++;
                int res = rand() % total;
                if(res == 0)
                    ret = i;
            }
        }
        return ret;
    }
private:
    vector<int> v;
    
};

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