1 class Solution {2     func isValid(_ s: String) -> Bool {3         //字符串为空返回true4         if s.isEmpty5         {6             return true7         }8         else9         {
10             //或字符串的字符个数为奇数个直接排除
11             if s.count%2==1
12             {
13                 return false
14             }
15         }
16         //创建字典对照表
17         var map:[Character:Character]=[")":"(","}":"{","]":"["]
18         //偶数符串若第一个字符就是右边的符号则直接排除
19         if  map[s[s.startIndex]] != nil
20         {
21             return false
22         }
23         //创建字符串堆栈
24         var stackOfString=Stack<Character>()
25         //遍历字符串
26         for char in s
27         {
28             //为右侧符号，且查询字典对应堆栈中最后一个元素
29             if map[char] != nil && map[char]==stackOfString.GetLastElement()
30             {
31                 //出栈
32                 stackOfString.pop()
33             }
34             else
35             {
36                 //入栈
37                 stackOfString.push(char)
38             }
39         }
40         return stackOfString.count()==0
41     }
42     //堆栈的泛型通用版本
43     struct Stack<Element> {
44         var items = [Element]()
45         //入栈
46         //mutating 关键字修饰方法是为了能在该方法中修改 struct 或是 enum 的变量
47         mutating func push(_ item: Element) {
48             items.append(item)
49         }
50         //出栈
51         mutating func pop() -> Element {
52             return items.removeLast()
53         }
54         //返回堆栈中的元素个数
55         mutating func count()->Int
56         {
57             return items.count
58         }
59         //获取最后一个元素
60         mutating func GetLastElement()->Element
61         {
62             return items[items.count-1]
63         }
64     }
65 }

 1 class Solution {2     func isValid(_ s: String) -> Bool {3         var opened = ""4         for char in s { 5             if char == "(" {6                 opened += ")" 7             } else if char == "{" {8                 opened += "}"9             } else if char == "[" {
10                 opened += "]"
11             } else {
12                 if let prevVal = opened.last, prevVal == char {
13                     opened.removeLast()
14                 } else {
15                     return false
16                 }
17             }
18         }
19         return opened.isEmpty
20     }
21 }