class Solution {

private:

    vector<string> coll;

    void helper(string s, int left, int right){

        if(left > right || left <  || right < )

            return;

        if( == left &&  == right){

            coll.push_back(s);

            return;

        }

        string lString = s, rString = s;

        helper(lString += '(', left - , right);

        helper(rString += ')', left, right - );

    }

public:

    vector<string> generateParenthesis(int n) {

        string s;

        helper(s, n, n);

        return coll;

    }

};