public static String getSmallestAndLargest(String s, int k) {
        String smallest = "";
        String largest = "";
        java.util.ArrayList<String> array = new java.util.ArrayList<String>();
                
        for (int i=0; i <= (s.length() - k); i++) {
            array.add(s.substring(i, i+k));
        }
        
        java.util.Collections.sort(array); 

        smallest = array.get(0);
        largest = array.get(array.size() - 1);
     
        
        return smallest + "\n" + largest;
    }