Strobogrammatic Number II

Source:

• leetcode: 247

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"].

Java

//backtracking

public List<String> findStrobogrammatic(int n) {
    return helper(n, n);
}

List<String> helper(int n, int m) {
    if (n == 0) return new ArrayList<String>(Arrays.asList(""));
    if (n == 1) return new ArrayList<String>(Arrays.asList("0", "1", "8"));

    List<String> list = helper(n - 2, m);
    List<String> res = new ArrayList<String>();

    for (int i = 0; i < list.size(); i++) {
        String s = list.get(i);
        if (n != m) res.add("0" + s + "0");

        res.add("1" + s + "1");
        res.add("6" + s + "9");
        res.add("8" + s + "8");
        res.add("9" + s + "6");
    }
    return res;
}