[LeetCode 题解]： Valid Parentheses

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.

题解：典型的STL stack 应用。注意边界条件。

 1 class Solution {
 2 public:
 3     bool isValid(string s) {
 4         stack<char> st;
 5         if(s.size()<=1 || s.size()%2) return false;
 6 
 7         for(int i=0;i<s.size();i++)
 8         {
 9             if(s[i]=='(' || s[i]=='[' || s[i]=='{') st.push(s[i]);
10             else if(st.empty())
11             {
12                 return false;
13             }
14             else
15             {
16                 if(  (s[i]==')' && st.top()=='(') ||
17                      (s[i]==']' && st.top()=='[') ||
18                      (s[i]=='}' && st.top()=='{') )
19                      st.pop();
20                 else
21                     return false;
22             }
23         }
24         if(!st.empty()) return false;
25         else return true;        
26     }
27 };

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