leetcode454 4Sum II

"""
Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.
To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.
Example:
Input:
A = [ 1, 2]
B = [-2,-1]
C = [-1, 2]
D = [ 0, 2]
Output:
2
Explanation:
The two tuples are:
1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0
"""
"""
I feel my IQ is screwed like a shit by this answer
将A+B的和记录在dict中。key为sum，value为sum出现的次数
依次求-(C+D)，判断是否在dict中。如果在
res 加上value
"""
class Solution:
    def fourSumCount(self, A, B, C, D):
        import collections
        res = 0
        dic = {}
        for a in A:
            for b in B:
                if (a+b) in dic:
                    dic[a+b] += 1
                else:
                    dic[a+b] = 1
        for c in C:
            for d in D:
                if -(c+d) in dic:
                    res += dic[-c-d]
        return res
if __name__ == '__main__':
    ans = Solution()
    A = [1, 2]
    B = [-2, -1]
    C = [-1, 2]
    D = [0, 2]
    print(ans.fourSumCount(A, B, C, D))