Valid Parentheses

    bool isValid(string str) {
        size_t n = str.length();
        stack<char>s;
        for (int i = 0; i < n; ++i)
        {
            //要注意及时检查栈是否为空，因为不能对空栈调用top函数
            if (s.empty() || str[i] == ‘(‘ || str[i] == ‘{‘ || str[i] == ‘[‘)
                s.push(str[i]);
            else
            {
                char c = s.top();
                if ( (c== ‘(‘ && str[i] == ‘)‘)
                    || (c== ‘[‘ && str[i] == ‘]‘)
                    || (c== ‘{‘ && str[i] == ‘}‘) )
                    s.pop();
                else
                    return false;
            }
        }
        return (s.empty()==true)?true:false;
    }