［LeetCode］454. 4Sum II

454. 4Sum II

题意：从给定的数组中统计四个数相加和为0的个数

思路：用哈希表来保存，四个数分成两部分，统计双方和的个数，最后相加。

import collections
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
        """
        number = collections.defaultdict(int)
        l = len(A)
        for i in range(l):
            for j in range(l):
                number[A[i]+B[j]] += 1
        res = 0
        for i in range(l):
            for j in range(l):
                neg = -1 * (C[i]+D[j])
                res += number[neg]
        return res