1512. Number of Good Pairs - Explanation

class Solution:
    def numIdenticalPairs(self, nums: List[int]) -> int:
        res = 0
        for i in range(len(nums)):
            for j in range(i + 1, len(nums)):
                if nums[i] == nums[j]:
                    res += 1
        return res

class Solution:
    def numIdenticalPairs(self, nums: List[int]) -> int:
        count = Counter(nums)
        res = 0
        for num, c in count.items():
            res += c * (c - 1) // 2
        return res

class Solution:
    def numIdenticalPairs(self, nums: List[int]) -> int:
        count = defaultdict(int)
        res = 0
        for num in nums:
            res += count[num]
            count[num] += 1
        return res