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)

Analysis: DFS. We can optimize a little bit during the recursion to avoid compute some unnecessary cases. Note something like "10.020.255.255" is meaningless. 

public class Solution {
    public void restoreIpAddress(String s, ArrayList<String> res, StringBuilder tem, int len, int level) {
        if(level == 4) {
            if(len==s.length()) res.add(tem.toString());
            return;
        }

        for(int i=len+1; i<=len+3; i++) {
            if(i<=s.length() && s.length()-i>=3-level && s.length()-i<=(3-level)*3) {   // optimaization
                String current = s.substring(len, i);
                if(Integer.valueOf(current)>=0 && Integer.valueOf(current)<=255) {
                    if(current.length()>1 && current.charAt(0)=='0') continue;  // cannot start from a '0'
                    tem.append(current);
                    if(level!=3) tem.append(".");
                    restoreIpAddress(s, res, tem, i, level+1);
                    if(level!=3) tem.delete(tem.length()-1-(i-len), tem.length());
                    else tem.delete(tem.length()-(i-len), tem.length());
                }
            }
        }
        return;
    }
    
    public ArrayList<String> restoreIpAddresses(String s) {
        ArrayList<String> res = new ArrayList<String>();
        StringBuilder tem = new StringBuilder();
        restoreIpAddress(s, res, tem, 0, 0);    // the init value of level should be 0!!!
        return res;
    }
}