[leetcode]Restore IP Addresses

 1 public class Solution {
 2         public List<String> restoreIpAddresses(String s) {
 3         List<String> result = new ArrayList<String>();
 4         if(s == null || s.length() < 4) return result;
 5         dfs(result,new StringBuilder(),s,0);
 6         return result;
 7     }
 8     
 9     private void dfs(List<String> result,StringBuilder pre,String suffix,int n){
10     if(n > 3 || (n == 3 && ( suffix.length() == 0 || suffix.length() > 3 || Integer.valueOf(suffix) > 255 || (suffix.length() > 1 && suffix.charAt(0) == ‘0‘))))return;
11         if(n == 3 && suffix.length() <= 3){
12             result.add(pre + suffix);
13             return;
14         }
15         for(int i = 0; i < Math.min(3, suffix.length()); i++){
16             StringBuilder oldPre = new StringBuilder(pre);
17             String tem = suffix.substring(0, i + 1);
18             int tem2Int = Integer.valueOf(tem);
19             if(tem2Int > 255 || (tem.length() > 1 && tem2Int < 10)) return;
20             pre = pre.append(tem).append(‘.‘);
21             n++;
22             dfs(result, pre, suffix.substring(i + 1), n);
23             pre = oldPre;
24             n--;
25         }
26     }
27 }