398. Random Pick Index -------M

 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);

A:

class Solution {
public:
    Solution(vector<int>& nums) {
        for(int i =0;i<nums.size();i++){
            M[nums[i]].push_back(i);
        }
    }
    
    int pick(int target) {
        auto& tmp = M[target];
        int size = tmp.size();
        int index = rand()% size;
        return tmp[index];
    }
private:
    unordered_map<int, vector<int>> M;
};

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