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

• 3-loop divides the string s into 4 substring: s1, s2, s3, s4. Check if each substring is valid.

Code

public class Solution {
    public List<String> restoreIpAddresses(String s) {
        List<String> res = new ArrayList<String>();
        int len = s.length();
        for (int i=1; i<4 && i<len-2;i++){
            for (int j=i+1; j<i+4 && j<len-1;j++){
                for (int k=j+1; k<j+4 && k<len ;k++){
                    String s1 = s.substring(0,i);
                    String s2 = s.substring(i,j);
                    String s3 = s.substring(j,k);
                    String s4 = s.substring(k,len);
                    if (isValid(s1) && isValid(s2) && isValid(s3) && isValid(s4)){
                        res.add(s1+"."+s2+"."+s3+"."+s4);
                    }
                }
            }
        }
        return res;
    }

    private boolean isValid(String s){
        int len = s.length();
        if (len>3 || len==0 || (s.charAt(0)=='0'&& len>1) || Integer.parseInt(s)>255){
            return false;
        }
        else return true;
    }
}

Reference

Leetcode