398. Random Pick Index

Given an integer array nums 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.

Implement the Solution class:

• Solution(int[] nums) Initializes the object with the array nums.
• int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i’s, then each index should have an equal probability of returning.

class Solution {
    unordered_map<int, vector<int>> m;
public:
    Solution(vector<int>& nums) {
        for(int i = 0; i < nums.size(); i++) {
            m[nums[i]].push_back(i);
        }
    }

    int pick(int target) {
        auto v = m[target];
        auto idx = rand()%m.size();

        return v[idx];
    }
};

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