Restore IP Addresses

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)

public class Solution {    public List
 
   restoreIpAddresses(String s) {        List
  
    res=new LinkedList
   
    ();        if(s==null || s.length()<=3) return res;        dfs(0,0,new String(""),s,res);        return res;    }    public void dfs(int start,int points,String IPstr,String s,List
    
      res){        if(start==s.length()&& points==4){            IPstr=IPstr.substring(0,IPstr.length()-1);            res.add(IPstr);            return;        }        if(points>=4) return;        if(s.length()-start<4-points) return;        if(s.length()-start>(4-points)*3) return;        if(start+1<=s.length()){            String sub1=s.substring(start,start+1);            dfs(start+1,points+1,IPstr+sub1+".",s,res);        }        if(start+2<=s.length()){            String sub2=s.substring(start,start+2);            if(sub2.compareTo("10")>=0 && sub2.compareTo("99")<=0){                dfs(start+2,points+1,IPstr+sub2+".",s,res);            }        }        if(start+3<=s.length()){            String sub3=s.substring(start,start+3);            if(sub3.compareTo("100")>=0 && sub3.compareTo("255")<=0){                dfs(start+3,points+1,IPstr+sub3+".",s,res);            }        }    }}