public List<String> generateParenthesis(int n) {

        List<String> res = new ArrayList<>();

        gen(res, n, n, "");

        return res;

    }

    private void gen(List<String> res, int left, int right, String str) {

        if (left == 0 && right == 0) {

            res.add(str);

            return;

        }

        if (left > 0) {

            gen(res, left - 1, right, str + "(");

        }

        if(right > left){

            gen(res, left, right - 1,str + ")");

        }

    }