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 class Solution {
    public List<String> restoreIpAddresses(String s) {
    	List<String> res = new ArrayList<>();
        String temp = new String();
        if (s.length()<4 || s.length()>12) {
        	return res;
        }
        dfs(res, temp, 0, s);
        return res;
    }

	private void dfs(List<String> res, String temp, int index, String s) {
		if (index==3 && isValid(s)) {
			res.add(temp+s);
			return;
		}
		for (int i = 0; i<3 && i<s.length()-1; i++) {
			String substring = s.substring(0, i+1);
			if (isValid(substring)) {
				dfs(res, temp+substring+'.', index+1, s.substring(i+1, s.length()));
			}
		}
	}

	private boolean isValid(String s) {
		if (s.charAt(0) == '0') {
			return s.equals("0");
		}
		int temp = Integer.parseInt(s);
		if (temp<=255 && temp>0) {
			return true;
		} else {
			return false;
		}
	}
}