CODE 40: 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)

public ArrayList<String> restoreIpAddresses(String s) {// Start typing your Java solution below// DO NOT write main() functionif (null == s) {return new ArrayList<String>();} else if (s.length() <= 3) {return new ArrayList<String>();}return dfs(s, 0, 0);}ArrayList<String> dfs(String s, int start, int th) {if (th < 3 && start >= s.length() - 3 + th) {return null;} else if (th == 3 && start < s.length() - 3) {return null;} else if (th == 3 && start >= s.length() - 3) {if (Integer.valueOf(s.substring(start)) > 255|| (s.substring(start).length() > 1 && s.substring(start).startsWith("0"))) {return null;}ArrayList<String> strings = new ArrayList<String>();strings.add(s.substring(start));return strings;}ArrayList<String> news = new ArrayList<String>();for (int i = 1; i <= 3; i++) {if (start + i < s.length()) {String tmp = s.substring(start, start + i);if (Integer.valueOf(tmp) > 255|| (tmp.length() > 1 && tmp.startsWith("0"))) {continue;}ArrayList<String> strs = dfs(s, start + i, th + 1);if (null != strs) {for (String str : strs) {news.add(tmp + "." + str);}}}}return news;}