题目：

题解：

class Solution:
    def fourSumCount(self, A: List[int], B: List[int], C: List[int], D: List[int]) -> int:
        countAB = collections.Counter(u + v for u in A for v in B)
        ans = 0
        for u in C:
            for v in D:
                if -u - v in countAB:
                    ans += countAB[-u - v]
        return ans