1 class Solution {
 2     func isAdditiveNumber(_ num: String) -> Bool {
 3         let array = [Character](num)
 4         let n = array.count
 5         if n < 3 { return false }
 6
 7         func helper(s1: String, s2: String, remain: String) -> Bool {
 8             let one = Int(s1)!
 9             let two = Int(s2)!
10             let next = "\(one + two)"
11             if next == remain { return true }
12             var newRemain = remain
13             if remain.hasPrefix(next) {
14                 let range = remain.range(of: next)!
15                 newRemain.replaceSubrange(range, with: "")
16                 return helper(s1: s2, s2: next, remain: newRemain)
17             } else {
18                 return false
19             }
20         }
21
22         //设第一个数是以i结尾的，第二个数是以j结尾的
23         for i in 0 ... (n - 1)/2 - 1 { //i只能取到一半以下的值
24             let one = String(array[0...i])
25             if one != "0" && one.hasPrefix("0") { continue }
26             for j in i + 1 ... n - 2 - i { //j至少取一位，且 n - j >= i
27                 let two = String(array[i+1 ... j])
28                 if two != "0" && two.hasPrefix("0") { continue }
29                 let remain = String(array[j+1 ... n-1])
30                 if remain != "0" && remain.hasPrefix("0") { continue }
31                 if helper(s1: one, s2: two, remain: remain) {
32                     return true
33                 }
34             }
35         }
36         return false
37     }
38 }

 1 class Solution {
 2     func isAdditiveNumber(_ num: String) -> Bool {
 3
 4         if num.count < 3 {
 5             return false
 6         }
 7
 8         let numArr = Array(num)
 9
10         for i in 1..<numArr.count-1 {
11             for j in i+1..<numArr.count {
12                 let f = String(numArr[0..<i])
13                 let s = String(numArr[i..<j])
14                 var fn = Int(f)!
15                 var sn = Int(s)!
16                 var ln = fn + sn
17
18                 var l = "\(ln)"
19                 var total = f + s + l
20
21                 if (f.count > 1 && f.first == "0" )||(s.count > 1 && s.first == "0") {
22                     continue
23                 }
24                 while total.count < num.count {
25                     let totalArr = Array(total)
26                     if totalArr != Array(numArr[0..<totalArr.count]) {
27                         break
28                     }
29                     fn = sn
30                     sn = ln
31                     ln = fn + sn
32                     l = "\(ln)"
33                     total += l
34                 }
35                 if total == num {
36                     return true
37                 }
38             }
39         }
40         return false
41     }
42 }