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)

Understand the problem:
The problem gives a string containing only digits, restore by returning all possible valid IP addresses combinations. 

So the crux of the problem is to understand "what is a valid IP address"? A valid IP address has the format of xxx.xxx.xxx.xxx, where xxx is a number from 0 to 255. Note that there are some reserved IP addresses, but this problem only asks for the valid structure of the IP addresses. Therefore a valid IP address could range from 0.0.0.0 to 255.255.255.255. 

Recursive Solution:
This is another permutation and combination problem. 

Code (Java):

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public class Solution {     public List<String> restoreIpAddresses(String s) {         List<String> result = new ArrayList<String>();                   if (s == null || s.length() < 4) {             return result;         }                   restoreHelper(s, 0, 1, "", result);                   return result;     }           private void restoreHelper(String s, int start, int segment, String curr, List<String> result) {         if (start >= s.length()) {             return;         }                   if (segment == 4) {             if (isValid(s.substring(start))) {                 result.add(curr + "." + s.substring(start));             }             return;         }                   for (int i = 1; i < 4 && start + i < s.length(); i++) {             String temp = s.substring(start, start + i);             if (isValid(temp)) {                 if (segment == 1) {                     restoreHelper(s, start + i, segment + 1, temp, result);                 } else {                     restoreHelper(s, start + i, segment + 1, curr + "." + temp, result);                 }             }         }     }           private boolean isValid(String str) {         if (str == null || str.length() > 3) {             return false;         }                   int num = Integer.parseInt(str);         if (str.charAt(0) == '0' && str.length() > 1) {             return false;         }                   if (num >= 0 && num <= 255) {             return true;         }                   return false;     } }