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)

import java.util.ArrayList;
  
public class Solution {
    private void restoreIpAddresses(String s, ArrayList<String> result, ArrayList<String> ip, int start) {
        if (start == s.length() && ip.size() == 4) {
            result.add(ip.get(0) + "." + ip.get(1) + "." + ip.get(2) + "." + ip.get(3));
            return;
        }
          
        if (s.length() - start > 3 * (4 - ip.size())) return;
        if (s.length() - start < 4 - ip.size()) return;
        int num = 0;
        for (int i = start; i < start + 3 && i < s.length(); i++) {
            num = num * 10 + (s.charAt(i) - '0');
            if (num > 255) {
                break;
            }
              
            if (i > start && s.charAt(start) == '0') break;
              
            ip.add(s.substring(start, i + 1));
            restoreIpAddresses(s, result, ip, i + 1);
            ip.remove(ip.size() - 1);
        }
    }
      
    public ArrayList<String> restoreIpAddresses(String s) {
        ArrayList<String> result = new ArrayList<>();
        ArrayList<String> ip = new ArrayList<>();
        if (s == null) return result;
        restoreIpAddresses(s, result, ip, 0);
        return result;
    }
}