# 【LeetCode】 1512. Number of Good Pairs

## Description
> Given an array of integers `nums`, return the number of good pairs.
A pair `(i, j)` is called good if `nums[i] == nums[j]` and `i` < `j`.

> Constraints:
> * `1 <= nums.length <= 100`
> * `1 <= nums[i] <= 100`

## Example:

Example 1:

Input: nums = [1,2,3,1,1,3]
Output: 4
Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.

Example 2:

Input: nums = [1,1,1,1]
Output: 6
Explanation: Each pair in the array are good.

Example 3:

Input: nums = [1,2,3]
Output: 0

## Solution

* 去計算每個數字以前出現過幾次，good pairs 就有多少
* 使用 map 實作

### Code

class Solution {
public:
    int numIdenticalPairs(vector<int>& nums) {
        map<int, int> counter;
        int ans = 0;
        for(int i = 0; i < nums.size(); i++)
        {
            ans += counter[nums[i]];
            counter[nums[i]]++;
        }
        return ans;
    }
};

###### tags: `LeetCode` `C++`