ArrayList<String> substrings = new ArrayList<>();
    for(int i=0;i<=s.length()-k;i++){
        String sub = s.substring(i,i+k);
        substrings.add(sub);  
    }
    smallest = substrings.get(0);
    largest = substrings.get(0);
    for(int j=1;j<substrings.size();j++){
        String currentword = substrings.get(j);
        if(currentword.compareTo(smallest)<0){
            smallest=currentword;
        }
        if(currentword.compareTo(largest)>0){
            largest=currentword;
        }
    }
     return smallest + "\n" + largest;
}