public static boolean isSubstring(String s1, String s2) {
        if (s1.length() < s2.length() || s1.length() == 0) {
            return false;
        }
        if (s2.length() == 0) {
            return true;
        }
        if (substringFromBeginning(s1,s2)){
            return true;
        }
        return isSubstring(s1.substring(1), s2);
    }

    private static boolean substringFromBeginning(String s1, String s2) {
        if (s2.length() == 0){
            return true;
        }
        if (s1.length() == 0){
            return false;
        }
        if (s1.charAt(0) == s2.charAt(0) && substringFromBeginning(s1.substring(1), s2.substring(1))){
            return true;
        }
        return false;
    }