2025-08-16 10:53:07 +0000 UTC

class Solution:
    def countQuadruplets(self, nums: List[int]) -> int:
        count = 0
        numbers = defaultdict(list)
        length = len(nums)
        for i in range(length - 1):
            for j in range(i + 1, length):
                numbers[nums[i] + nums[j]].append(j)
        for i in range(2, length - 1):
            for j in range(i + 1, length):
                target = nums[j] - nums[i]
                for idx in numbers[target]:
                    if i > idx:
                        count += 1
        return count