Random Pick Index -- LeetCode

[Question]

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);

[Analysis]
This is a typical Reservior Sampling problem. Usually, it is used to when data set is too large to feed into memory. The time complexity of pick() is O(N). The additional space is O(1).

[Solution]
class Solution {
    vector<int> n;
public:
    Solution(vector<int> nums): n(nums) {
    }
 
    int pick(int target) {
        int count=0, res=-1;
        for (int i=0; i<n.size(); i++) {
            if (n[i]!=target) continue;
            if (rand() % (++count) ==0) res=i;
        }
        return res;
    }

};