Thank @asurana28 's solution, I implemented in in swift 3.0

```
class Solution {
func fourSumCount(_ A: [Int], _ B: [Int], _ C: [Int], _ D: [Int]) -> Int {
let length = A.count
var dic:[Int:Int] = [:]
for a in A {
for b in B {
let t = a+b
if let v = dic[t] {
dic[t] = v+1
} else {
dic[t] = 1
}
}
}
var res = 0
for c in C {
for d in D {
let t = c+d
if let v = dic[-t] {
res += v
}
}
}
return res
}
}
```