247. Strobogrammatic Number II

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"].

dfs

public class Solution {
    //dfs
    //考虑 0110的状况 主要是去掉0
    public List<String> findStrobogrammatic(int n) {
        List<String> member = new ArrayList<>();
        member = dfs(n, n);  
        return member;
    }
    public List<String> dfs(int n , int m) {
        if(n == 0) {
            ArrayList<String> member = new ArrayList<>();
            member.add("");
            return new ArrayList<>(member);
        }
        if(n == 1) {
            ArrayList<String> member = new ArrayList<>();
            member.add("0");
            member.add("1");
            member.add("8");
            return new ArrayList<>(member);
        }
        List<String> list = dfs(n - 2, m);
        ArrayList<String> newList = new ArrayList<String>(); // from bottom to top consider 0
        for(int i = 0 ; i< list.size(); i++){
            newList.add("1" + list.get(i) + "1");
            newList.add("6" + list.get(i) + "9");
            newList.add("9" + list.get(i) + "6");
            newList.add("8" + list.get(i) + "8");
            if(n != m) //去0 like 0110， 0880
                newList.add("0" + list.get(i) + "0");
        }
        return newList;
    }
    
}

from bottom to up consider 0 like n =4 0110,0000