Leetcode-Resotre 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)

Analysis:

This is a recursive problem. A string will be divided into four parts. For each part, we should determine whether it is a valid IP segment.

Solution:

 public class Solution {
     public List<String> restoreIpAddresses(String s) {
           List<String> res = new ArrayList<String>();
           if (s.length()==0) return res;

           res = restoreRecur(s,0,4);
           return res;
     }

     public List<String> restoreRecur(String s, int curIndex, int num){
         List<String> res = new ArrayList<String>();
         if (curIndex>=s.length()) return res;

         if (num==1){
            String temp = s.substring(curIndex,s.length());
            if (temp.length()>3) return res;
            int val = Integer.parseInt(temp);
            if (temp.length()==3 && val>=100 && val<=255){
                res.add(temp);
                return res;
            }
            if (temp.length()==2 && val>=10 && val<=99){
                res.add(temp);
                return res;
            }
            if (temp.length()==1){
                res.add(temp);
                return res;
            }
            return res;
         }

         if (curIndex+1>=s.length()) return res;
         int end = curIndex+3;
         if (curIndex+3>s.length())
             end = s.length();

         for (int i=curIndex+1;i<=end;i++){
             String temp = s.substring(curIndex,i);
             int val = Integer.parseInt(temp);
             List<String> nextRes = new ArrayList<String>();
             if (temp.length()==3 && val>=100 && val<=255){
                 nextRes = restoreRecur(s,i,num-1);
             }
             if (temp.length()==2 && val>=10 && val<=99){
                 nextRes = restoreRecur(s,i,num-1);
             }
             if (temp.length()==1){
                 nextRes = restoreRecur(s,i,num-1);
             }
             for (int j=0;j<nextRes.size();j++)
                 res.add(temp+"."+nextRes.get(j));
         }
         return res;       

     }
 }