32. Longest Valid Parentheses

class Solution {
public:
    int longestValidParentheses(string s) {

       stack<char> s1; //to store the character
       stack<int> s2; //to store the invalid character index
       s2.push(-1);

       int maxi = 0;

       for(int i=0; i<s.length(); i++)
       {    
           //we consider first opening bracket is a problem character     //because we assume that we can't get it's pair(closing barcket)
           if(s[i] == '(')
           {
                s1.push(s[i]);
                s2.push(i);
           }
           else
           {    
        //if character stack is empty means that is a problem character
               if(s1.empty())
               {
                   s2.push(i);
               }
               else //we get a pair
               {
                   s1.pop();
                   s2.pop();

                   int a = s2.top();//last problem index

                   if(i-a > maxi) //calculate the maximum length
                    maxi = i - a;
               }
           }
       } 


       return maxi;
    }
};