class Solution {
public:
    int longestValidParentheses(string s) {
        stack< pair<char,int> >a;
        int ans=0;
        a.push({'[',0});
        for(auto i:s)
        {
        if(i=='(')a.push({i,0});
        else if(i==')'){
            if(a.size()>1 && a.top().first=='('){
                int cnt=a.top().second+2;
                a.pop();
                pair<char,int> op=a.top();
                a.pop(); op.second+=cnt;
                a.push(op);

            }
            else a.push({i,0});
        }

        }
        while(a.size()){
            ans=max(ans, a.top().second);
            a.pop();
        }
        return ans;
    }
};