Python | Leetcode Python题解之第454题四数相加II
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