Valid Parenthesis String

class Solution {
public:
    bool checkValidString(string s) {
        stack <int> open ; 
        stack <int> star ; 
        for(int i = 0 ; i < s.size() ; i++)
        {
            if(s[i] == '(')
            {
                open.push(i)  ;
            }
            else if(s[i] == '*')
            {
                star.push(i) ; 
            }
            else{
                if(!open.empty())
                {
                    open.pop() ; 
                }
                else if(!star.empty() && star.top() < i)
                {
                    star.pop() ; 
                }
                else
                {
                    return false ; 
                }
            }
        }
        if(open.size() != 0)
        {
            while(!open.empty())
            {
                int o = open.top() ; 
                open.pop() ; 
                if(star.empty() || star.top() < o )
                return false ; 
                else
                star.pop() ; 
            }
        }
       return true  ;  
    }
};