LeetCode OJ：Valid Parentheses（有效括号）

Given a string containing just the characters ‘(‘, ‘)‘, ‘{‘, ‘}‘, ‘[‘ and ‘]‘, determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

简单的堆栈问题，代码如下：

 1 class Solution {
 2 public:
 3     bool isValid(string s) {
 4         if(!s.size())
 5             return true;
 6         char c;
 7         stack<char> stk;
 8         for(int i = 0; i < s.size(); ++i){
 9             if(s[i] == ‘[‘ || s[i] == ‘{‘ || s[i] == ‘(‘){
10                 stk.push(s[i]);
11                 continue;
12             }else if(s[i] == ‘]‘){
13                 if(stk.empty())
14                     return false;
15                 c = stk.top();
16                 stk.pop();
17                 if(c != ‘[‘)
18                     return false;
19             }else if(s[i] == ‘}‘){
20                 if(stk.empty())
21                     return false;
22                 c = stk.top();
23                 stk.pop();
24                 if(c != ‘{‘)
25                     return false;
26             }else{
27                 if(stk.empty())
28                     return false;
29                 c = stk.top();
30                 stk.pop();
31                 if(c != ‘(‘)
32                     return false;
33             }
34         }
35         if(stk.empty())
36             return true;
37         return false;
38     }
39 };