week6 22. Generate Parentheses

public class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> res = new ArrayList<>();
        helper (res, new String(), n, 0);
        return res;
    }
    public void helper (List<String> res, String str, int n, int counter){
        if(counter == 0 && str.length() == 2 * n){
            res.add(new String(str));
            return;
        }
        if(counter < 0 || counter > n || str.length() > 2*n){
            return;
        }
        if(counter > 0 && counter < n){
            helper(res, str + "(", n, counter + 1);
            helper(res, str + ")", n, counter - 1);
        }else if (counter == 0){
            helper(res, str + "(", n, counter + 1);
        }else {
            helper(res, str + ")", n, counter - 1);
        }
        return;
    }
}