leetcode454 4Sum II

 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))