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 = 0for u in C:for v in D:if -u - v in countAB:ans += countAB[-u - v]return ans