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,

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;      }  }