20. Valid Parentheses

static int wing=[]()
{
     std::ios::sync_with_stdio(false);
     cin.tie(NULL);
     return 0;
}();

class Solution 
{
public:
    bool isValid(string s) 
    {
        vector<char> stack;
        if(s.length()==0)
            return true;
        if(s[0]==')'||s[0]==']'||s[0]=='}')
            return false;
        for(char c:s)
        {
            switch(c)
            {
                case'(':  stack.push_back(c);break;
                case'[':  stack.push_back(c);break;
                case'{':  stack.push_back(c);break;
                case')':  
                    if(stack.back()!='(')  
                        return false;
                    else
                        stack.pop_back();
                    break;
                case']':  
                    if(stack.back()!='[')
                        return false;
                    else
                        stack.pop_back();
                    break;
                case'}':  
                    if(stack.back()!='{')
                        return false;
                    else
                        stack.pop_back();
                    break;
            }
        }
        return stack.empty();
    }
};

借助一个栈来判断，基础题，无压力。