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 {
    /**
     * @param s the IP string
     * @return All possible valid IP addresses
     */
    public ArrayList<String> restoreIpAddresses(String s) {
        // Write your code here
        ArrayList<String> res = new  ArrayList<String>();
        ArrayList<String> list = new ArrayList<String>();
        if (s == null || s.length() == 0) {
            return res;
        }
        
        helper(res, s, list, 0);
        return res;
        
    }
    
    private void helper( ArrayList<String> res, String s, ArrayList<String> list, int start) {
        if (list.size() == 4) {
            if (start != s.length()) {
              return;
            }
            
            StringBuilder sb = new StringBuilder();
            for (String tmp : list) {
                sb.append(tmp);
                sb.append(".");
            }
            sb.deleteCharAt(sb.length() - 1);
            res.add(sb.toString());
            return;
        }
        
        for (int i = start; i < s.length() && i < start + 3; i++) {
            String item = s.substring(start, i + 1);
            if (isValid(item)) {
            list.add(item);
            helper(res, s, list, i + 1);
            list.remove(list.size() - 1);
            }
            }
    }
    
    private boolean isValid(String s) {
        if (s.charAt(0) == ‘0‘)  {
            return s.equals("0");
        }
        int digit = Integer.valueOf(s);
        return digit >= 0 && digit <= 255;
    }
}