vector<string> valid;
  void generate(string& s , int open,int close)
  {
    
    if(open==0 && close==0)
    {
        valid.push_back(s);
        return;
    }
    if(open>0)
    {
        s.push_back('(');
        generate(s,open-1,close);
        s.pop_back();
    }
    if(close>0 && open<close)
    {
        s.push_back(')');
        generate(s,open,close-1);
        s.pop_back();
    }
  }