[Swift]LeetCode293. 翻转游戏 $ Flip Game

You are playing the following Flip Game with your friend: Given a string that contains only these two characters: + and -, you and your friend take turns to flip twoconsecutive "++" into "--". The game ends when a person can no longer make a move and therefore the other person will be the winner.

Write a function to compute all possible states of the string after one valid move.

For example, given s = "++++", after one move, it may become one of the following states:

[ "--++",  "+--+",  "++--"] 

If there is no valid move, return an empty list [].

---

您正在和您的朋友玩以下翻转游戏：给定一个仅包含这两个字符的字符串：+和-，您和您的朋友轮流将两个插入“++”翻转为“--”。游戏结束时，一个人不能再做一个动作，因此另一个人将是赢家。

写一个函数来计算一次有效移动后字符串的所有可能状态。

例如，给定s=“+++++”，移动一次后，它可能成为以下状态之一：

[ "--++",  "+--+",  "++--"] 

如果没有有效的移动，请返回空列表[]。

---

Solution:

 1 class Solution {
 2     func generatePossibleNextMoves(_ s:String) ->[String] {
 3         var res:[String] = [String]()
 4         var arrS:[Character] = Array(s)
 5         for i in 1..<s.count
 6         {
 7             if arrS[i] == "+" && arrS[i - 1] == "+"
 8             {
 9                 res.append(s.subString(0, i - 1) + "--" + s.subString(i + 1))
10             }
11         }
12         return res          
13     }
14 }
15 
16 extension String {
17     // 截取字符串：从index到结束处
18     // - Parameter index: 开始索引
19     // - Returns: 子字符串
20     func subString(_ index: Int) -> String {
21         let theIndex = self.index(self.endIndex, offsetBy: index - self.count)
22         return String(self[theIndex..<endIndex])
23     }
24     
25     // 截取字符串：指定索引和字符数
26     // - begin: 开始截取处索引
27     // - count: 截取的字符数量
28     func subString(_ begin:Int,_ count:Int) -> String {
29         let start = self.index(self.startIndex, offsetBy: max(0, begin))
30         let end = self.index(self.startIndex, offsetBy:  min(self.count, begin + count))
31         return String(self[start..<end]) 
32     }
33 }