Given an input string, reverse the string word by word. A word is defined as a sequence of non-space characters.
The input string does not contain leading or trailing spaces and the words are always separated by a single space.
For example,
Given s = "the sky is blue",
return "blue is sky the".
Could you do it in-place without allocating extra space?

給定輸入字串，逐字反轉字串。單詞被定義為非空格字元序列。

輸入字串不包含前導或字尾空格，並且單詞總是由一個空格分隔。

例如，

給定  s = "the sky is blue",

返回 "blue is sky the".。

您能在不分配額外空間的情況下就地完成嗎？

 1 class Solution {
 2     func reverseWords(_ s: inout String){
 3         var left:Int = 0
 4         for i in 0...s.count
 5         {
 6             if
 
 i == s.count || s[i] == " "
 7             {
 8                 reverse(&s, left, i - 1)
 9                 left = i + 1
10             }
11         }
12         reverse(&s, 0, s.count - 1)
13     }
14     
15     func reverse(_ s: inout String,_ left:Int,_ right)
16     {
17         while
 
 (left < right)
18         {
19             var t:Character = s[left]
20             s[left] = s[right]
21             s[right] = t
22             left += 1
23             right -=1
24         }
25     }
26 }
27 
28 extension String {        
29     //subscript函式可以檢索陣列中的值
30     //直接按照索引方式擷取指定索引的字元
31     subscript (_ i: Int) -> Character {
32         //讀取字元
33         get {return self[index(startIndex, offsetBy: i)]}
34         
35         //修改字元
36         set
37         {
38             var str:String = self
39             var index = str.index(startIndex, offsetBy: i)
40             str.remove(at: index)
41             str.insert(newValue, at: index)
42             self = str
43         }
44     }
45 }