Leetcode: 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"].

Hint:

Try to use recursion and notice that it should recurse with n - 2 instead of n - 1.

Watch out for cornor case:

when n = 2, there shouldn't be "00" returned.

but when n = 4, which uses n =2 as recursion base, "1001" is an answer

class Solution {
    public List<String> findStrobogrammatic(int n) {
        return helper(n, n);
    }
    
    public List<String> helper(int n, int limit) {
        if (n <= 0) return Arrays.asList("");
        if (n == 1) return Arrays.asList("0", "1", "8");
        
        List<String> list = helper(n - 2, limit);
        
        List<String> res = new ArrayList<>();
        
        for (String str : list) {
            if (n != limit) {
                res.add("0" + str + "0");
            }
            res.add("1" + str + "1");
            res.add("6" + str + "9");
            res.add("9" + str + "6");
            res.add("8" + str + "8");
        }
        return res;
    }
}