1 class Solution {
 2     func canWin(_ s:String) -> Bool {
 3         var arrS:[Character] = Array(s)
 4         for i in 1..<s.count
 5         {
 6             if arrS[i] == "+" && arrS[i - 1] == "+" && !canWin(s.subString(0, i - 1) + "--" + s.subString(i + 1))
 7             {
 8                 return true
 9             }
10         }
11         return false          
12     }
13 }
14 
15 extension String {
16     // 截取字符串：从index到结束处
17     // - Parameter index: 开始索引
18     // - Returns: 子字符串
19     func subString(_ index: Int) -> String {
20         let theIndex = self.index(self.endIndex, offsetBy: index - self.count)
21         return String(self[theIndex..<endIndex])
22     }
23     
24     // 截取字符串：指定索引和字符数
25     // - begin: 开始截取处索引
26     // - count: 截取的字符数量
27     func subString(_ begin:Int,_ count:Int) -> String {
28         let start = self.index(self.startIndex, offsetBy: max(0, begin))
29         let end = self.index(self.startIndex, offsetBy:  min(self.count, begin + count))
30         return String(self[start..<end]) 
31     }
32 }

1 var sol = Solution()
2 print(sol.canWin("++++"))
3 //Print true