Leetcode: Restore IP addresses

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:
Given "25525511135",

return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

 1 public class Solution {
 2     public List<String> restoreIpAddresses(String s) {
 3         ArrayList<String> res = new ArrayList<String>();
 4         if (s == null || s.length() == 0) return res;
 5         helper(res, "", s, 1, 0);
 6         return res;
 7     }
 8     
 9     public void helper(ArrayList<String> res, String item, String s, int level, int index) {
10         if (index >= s.length()) return;
11         if (level == 4) {
12             String st = s.substring(index);
13             if (isValid(st)) {
14                 String f = String.format("%s.%s", item, st);
15                 res.add(f);
16             }
17             return;
18         }
19         for (int i=1; i<4&&(index+i<=s.length()); i++) {
20             String str = s.substring(index, index+i);
21             if (isValid(str)) {
22                 if (item.length() == 0) {
23                     helper(res, str, s, level+1, index+i);
24                 }
25                 else {
26                     String temp = String.format("%s.%s", item, str);
27                     helper(res, temp, s, level+1, index+i);
28                 }
29             }
30         }
31     }
32     
33     public boolean isValid(String str) {
34         if (str == null || str.length()>3) return false;
35         if (str.charAt(0) == ‘0‘ && str.length()>1) return false;
36         int num = Integer.parseInt(str);
37         if (num>=0 && num<=255) {
38             return true;
39         }
40         return false;
41     }
42 }