1 public class Solution {
 2     private boolean isValidIp(String ip){
 3         if(ip.charAt(0) == '0')
 4             return ip.equals("0");
 5         int digit = Integer.valueOf(ip);
 6         return digit >= 0 && digit <= 255;
 7     }
 8     private void restoreIpRecur(String s,List<String> answer,ArrayList<String> result){
 9         //result has more than four numbers but s is not empty, means we sperate s to more than 4 numbers
10         if(result.size()>=4 && !s.isEmpty())
11             return;
12         if(s.equals("")){
13             //if we sperate s exactly four valid numbers
14             if(result.size()==4){
15                 //found one IP
16                 String ip = new String();
17                 for(String ss:result)
18                     ip = ip + ss + ".";
19                 ip = ip.substring(0,ip.length()-1);
20                 answer.add(ip);
21             }
22             else {
23                 return;
24             }
25         }
26         //get 1,2,3 characters from s's head and put it into result,search what's left in s recursively
27         for(int i = 1;i<=3&&i<=s.length();i++){
28             String sub = s.substring(0,i);
29             if(isValidIp(sub)){
30                 result.add(sub);
31                 restoreIpRecur(s.substring(i), answer, result);
32                 result.remove(result.size()-1);
33             }
34         }
35         return;
36     }
37     public List<String> restoreIpAddresses(String s) {
38         ArrayList<String> answer = new ArrayList<String>();
39         ArrayList<String> result = new ArrayList<String>();
40         restoreIpRecur(s, answer, result);
41         return answer;
42
43     }
44 }