Given an array of integers nums.
A pair (i,j) is called good if nums[i] == nums[j] and i < j.
Return the number of good pairs.
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
Constraints:
1 <= nums.length <= 100
1 <= nums[i] <= 100
class Solution:
    def numIdenticalPairs(self, nums: List[int]) -> int:
        counter = collections.Counter(nums)
        answer = 0
        
        for k, v in counter.items():
            answer += v * (v - 1) // 2
        return answer
class Solution:
    def numIdenticalPairs(self, nums: List[int]) -> int:
        counter = collections.Counter(nums)
        answer = 0
        
        for k, v in counter.items():
            answer += v * (v - 1) // 2
        return answer