# 454. 4Sum II 四数相加 II

@TOC

## # 题目描述

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
``````

## # 解题方法

### # 字典

``````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 = collections.Counter(a + b for a in A for b in B)
return sum(AB[-c-d] for c in C for d in D)
``````

``````class Solution {
public:
int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
const int N = A.size();
unordered_map<int, int> AB;
unordered_map<int, int> CD;
for (int i = 0; i < N; ++i) {
for (int j = 0; j < N; ++j) {
++AB[A[i] + B[j]];
++CD[C[i] + D[j]];
}
}
int res = 0;
for (auto ab : AB) {
res += ab.second * CD[-ab.first];
}
return res;
}
};
``````

## # 日期

2018 年 3 月 7 日 2018 年 11 月 11 日 —— 剁手节快乐 2019 年 1 月 25 日 —— 这学期最后一个工作日