1512. Number of Good Pairs

https://leetcode.com/problems/number-of-good-pairs/

1. Use hashmap:

//4 ms    7.3 MB
class Solution {
public:
    int numIdenticalPairs(vector<int>& nums) {
        map<int, int> counter;
        for(int i=0; i < nums.size(); i++)
            counter[nums[i]]++;
        int result = 0;
        for(map<int, int>::iterator it = counter.begin(); it != counter.end(); it++)
            result += (it->second * (it->second-1))/2;
        return result;
    }
};