LeetCode Generate Parentheses

public class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> res = new ArrayList<String>();
        if(n <= 0){
            return res;
        }
        String str = new String();
        helper(n, n, str, res);
        return res;
    }
    private void helper(int left, int right, String str, List<String> res){
        if(left > right){ 
            //if left > right, it means remaining "(" is more than remaining ")"
            //This means already added more ")" than "(", which is illegal.
            return;
        }
        if(left == 0 && right == 0){
            res.add(str);
            return;
        }
        if(left > 0){
            helper(left-1, right, str+"(", res);
        }
        if(right > 0){
            helper(left, right-1, str+")", res);
        }
    }
}