 6/21/2017 - 10:17 AM

## 454. 4Sum II

1. 4Sum II
``````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
"""
ab_dict = {}
for i in range(len(A)):
for j in range(len(B)):
sum = A[i] + B[j]
ab_dict[sum] = ab_dict.get(sum, 0) + 1
res = 0
for i in range(len(C)):
for j in range(len(D)):
sum  = C[i] + D[j]
target = 0 - sum
res += ab_dict.get(target,0)
return res``````

https://leetcode.com/problems/4sum-ii/#/description

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 + B + C + D = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> A + B + C + D = 2 + (-1) + (-1) + 0 = 0
``````