public static String getSmallestAndLargest(String s, int k) { String smallest = s.substring(0,k); String largest = s.substring(0,k);

    for(int i=0;i<s.length()-k;i++){
        String current = s.substring(i,k+i);
        if(current.compareTo(smallest)<0){
            smallest = current;
        }

        if(current.compareTo(largest)>0){
            largest = current;
        }
    }

    return smallest + "\n" + largest;
}