public static String getSmallestAndLargest(String s, int k){
        List<String> subStrings = 
        IntStream.rangeClosed(0, s.length()-k)
                .mapToObj(i -> s.substring(i,i+k))
                .sorted(Comparator.naturalOrder())
                .collect(Collectors.toList());
            
        String smallest = subStrings.get(0);
        String largest = subStrings.get(subStrings.size()-1);
                
        return smallest+"\n"+largest;
    }