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)

[java]  view plain  copy

1. public class Solution {  
2.     public List<String> restoreIpAddresses(String s) {  
3.         List<String> res = new ArrayList<String>();       
4.         if(s.equals("")) return res;  
5.         Stack<String> stack = new Stack<String>();  
6.         helper(s, 4, res, stack);             
7.         return res;  
8.     }  
9.       
10.     void helper(String s,int count,List<String> res,Stack<String> stack){  
11.         if(count==0){             
12.             if(s.length()==0){                
13.                 String str = "";                  
14.                 for(int i=0;i<stack.size();i++){  
15.                     str += ".";  
16.                     str += stack.get(i);  
17.                 }                                 
18.                 res.add(str.substring(1));  
19.                 return;  
20.             }else{  
21.                 return;   
22.             }  
23.         }else{  
24.             for(int i=1;i<4;i++){  
25.                 if(s.length()>=i&&Integer.valueOf(s.substring(0,i))<=255){  
26.                     if(i==1||(i>=2&&s.charAt(0)!='0')){  
27.                         stack.add(s.substring(0,i));  
28.                         helper(s.substring(i), count-1, res, stack);  
29.                         stack.pop();  
30.                     }  
31.                 }  
32.             }  
33.         }  
34.     }  
35. }

原文链接http://blog.youkuaiyun.com/crazy__chen/article/details/46445081