python leetcode 454. 4Sum II

这类题目基本是用字典 以空间换时间

class Solution(object):
    def fourSumCount(self, A, B, C, D):
        """
        :type A: List[int]
        :type B: List[int]
        :type C: List[int]
        :type D: List[int]
        :rtype: int
        """
        dict1={}
        res=0
        for i in range(len(A)):
            for j in range(len(B)):
                if not A[i]+B[j] in dict1:
                    dict1[A[i]+B[j]] = 1
                else:
                    dict1[A[i]+B[j]]+=1
        for m in range(len(C)):
            for n in range(len(D)):
                if -(C[m]+D[n]) in dict1:
                    res+=dict1[-(C[m]+D[n])]
        return res