[leetcode]20. Valid Parentheses

class Solution {
public:
bool isValid(string s) {
int length=s.length();
if(length==0) return true;
if(length%2 != 0) return false;

char flag[6]={'(',')','{','}','[',']'};
stack<char> a;
a.push(s[0]);

for(int i=1; i<length; i++)
{
if(a.empty())
a.push(s[i]);
else
{
char tmp = a.top();
if( ((tmp==flag[0])&&(s[i]==flag[1])) || ((tmp==flag[2])&&(s[i]==flag[3])) || ((tmp==flag[4])&&(s[i]==flag[5])) )
a.pop();
else
a.push(s[i]);
}
}

if(a.empty())
return true;
else
return false;
}
};

Given a string containing just the characters '(',')','{','}','[',']', determine if the input string is valid.
The brackets must close in the correct order,"()" and "()[]{}" are all valid but "(]" and "([)]" are not valid

最开始看到这个问题以为是规律给出来的，比如说是这样的：(){}[]()，这样的理解是不对的，([])这样的结果也是对的

所以，用栈这种数据结构来处理这种问题是最方便的，代码如下