398.cpp

// simple.cpp
class Solution {
  unordered_map<int, int> hashmap;
  vector<int> nums;

public:
  Solution(vector<int> nums) {
    int n = nums.size();
    for (int i = 0; i < n; ++i)
      hashmap[nums[i]]++;
    this->nums = nums;
  }

  int pick(int target) {
    int index = rand() % hashmap[target];
    for (int i = 0; i < nums.size(); ++i)
      if (nums[i] == target)
        if (index-- == 0)
          return i;
  }
};

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