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):

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;
    }
}