leetcode 20

 1 class Solution {
 2 public:
 3     bool isValid(string s) {
 4         int n = s.length();
 5         if(n%2 != 0)
 6         {
 7             return false;
 8         }
 9         vector<char> stack;
10         if(s[0] != ‘(‘ && s[0] != ‘[‘ && s[0] != ‘{‘)
11         {
12             return false;
13         }
14         else
15         {
16             stack.push_back(s[0]);
17         }
18         int j = 1;
19         for(int i = 1; i < n; ++i)
20         {
21             if(s[i] == ‘(‘ || s[i] == ‘[‘ || s[i] == ‘{‘)
22             {
23                 stack.push_back(s[i]);
24             }
25             else if(s[i] == ‘)‘)
26             {
27                 if(stack.back() == ‘(‘)
28                 {
29                     stack.pop_back();
30                 }
31                 else
32                 {
33                     return false;
34                 }
35             }
36             else if(s[i] == ‘]‘)
37             {
38                 if(stack.back() == ‘[‘)
39                 {
40                     stack.pop_back();
41                 }
42                 else
43                 {
44                     return false;
45                 }
46             }
47             else if(s[i] == ‘}‘)
48             {
49                 if(stack.back() == ‘{‘)
50                 {
51                     stack.pop_back();
52                 }
53                 else
54                 {
55                     return false;
56                 }
57             }
58         }
59         if(stack.empty())
60         {
61             return true;
62         }
63         else
64         {
65             return false;
66         }
67     }
68 };