public static String getSmallestAndLargest(String s, int k) {
        String smallest = "";
        String largest = "";

        int a = 0, b = 0;
        for (int i=0; i<s.length()-(k-1); i++){
             String sub = s.substring(i, i+k);
             a = sub.compareTo(s.substring(a, a+k))>0 ? i : a;
             b = sub.compareTo(s.substring(b, b+k))<0 ? i : b;
        }
        smallest = s.substring(b, b+k);
        largest = s.substring(a, a+k);
        
        return smallest + "\n" + largest;
    }