leetcode-有效的括号

https://leetcode-cn.com/problems/valid-parentheses/description/

class Solution
{
public:
    bool isValid(string s)
    {
        stack<char> ss;
        //char stack[1000];
        //int top = 0;
        int len = s.length();
        int flag = 1;
        int i = 0;
        while(i<len && flag)
        {
            if(s[i] == '(' || s[i] == '[' || s[i] == '{')
                ss.push(s[i]);
            if(s[i] == ')')
                if(!ss.empty() && ss.top() == '(')
                    ss.pop();
                else
                    flag = 0;
            if(s[i] == ']')
                if(!ss.empty() && ss.top() == '[')
                    ss.pop();
                else
                    flag = 0;
            if(s[i] == '}')
                if(!ss.empty() && ss.top() == '{')
                    ss.pop();
                else
                    flag = 0;
            i++;
        }
        if(ss.empty() && flag == 1)
            return true;
        else
            return false;
    }
};

因为有pop()操作,所以需要先判断栈是否空