93. Restore IP Address

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)

思路：使用substring()

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;
    }

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