LeetCode(Google) 0247 Strobogrammatic Number 2

LeetCode 0247 Strobogrammatic Number 2

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).

Find all strobogrammatic numbers that are of length = n.

For example,
Given n = 2, return ["11","69","88","96"].

public class StrobogrammaticNumberII {  
    public List findStrobogrammatic(int n) {  
        return helper(n, n);  
    }  
  
    private List helper(int n, int m) {  
        if (n == 0) return new ArrayList<>(Arrays.asList(""));  
        if (n == 1) return new ArrayList<>(Arrays.asList("0", "1", "8"));  
        List list = helper(n - 2, m);  
        List result = new ArrayList<>();  
        for (int i = 0; i < list.size(); i++) {  
            String s = list.get(i);  
            if (n != m)     result.add("0" + s + "0");  
            result.add("1" + s + "1");  
            result.add("6" + s + "9");  
            result.add("8" + s + "8");  
            result.add("9" + s + "6");  
        }  
        return result;  
    }  
}