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);

class Solution {
private:
    map<int, vector<int>> mp;
public:
    Solution(vector<int>& nums) {
        for (int i = 0; i < nums.size(); i++) {
            mp[nums[i]].push_back(i);
        }
    }
   
    int pick(int target) {
        map<int, vector<int>>::iterator it = mp.find(target);
        if (it != mp.end()) {
            int size = (it -> second).size();
            return (it -> second)[rand() % size];
        }
        return -1;
    }
};

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

======= Without MAP ======
class Solution { public: Solution(vector<int>& nums) : m_nums(nums) { std::ios::sync_with_stdio(false); std::cin.tie(NULL); } int pick(int target) { int len = 0; int res = -1; for (size_t i = 0; i < m_nums.size(); ++i) { if (m_nums[i] == target) { if (len == 0) { res = i; } else { int r = rand() % (len + 1); if (r == 0) { res = i; } } ++len; } } return res; } private: vector<int>& m_nums; };