1 """
2 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.
3 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.
4 Example:
5 Input:
6 A = [ 1, 2]
7 B = [-2,-1]
8 C = [-1, 2]
9 D = [ 0, 2]
10 Output:
11 2
12 Explanation:
13 The two tuples are:
14 1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
15 2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0
16 """
17 """
18 I feel my IQ is screwed like a shit by this answer
19 将A+B的和记录在dict中。key为sum，value为sum出现的次数
20 依次求-(C+D)，判断是否在dict中。如果在
21 res 加上value
22 """
23 class Solution:
24 def fourSumCount(self, A, B, C, D):
25 import collections
26 res = 0
27 dic = {}
28 for a in A:
29 for b in B:
30 if (a+b) in dic:
31 dic[a+b] += 1
32 else:
33 dic[a+b] = 1
34 for c in C:
35 for d in D:
36 if -(c+d) in dic:
37 res += dic[-c-d]
38 return res
39 if __name__ == '__main__':
40 ans = Solution()
41 A = [1, 2]
42 B = [-2, -1]
43 C = [-1, 2]
44 D = [0, 2]
45 print(ans.fourSumCount(A, B, C, D))
