I create different solution, but passed tests: public static String getSmallestAndLargest(String s, int k) {

    ArrayList<String> subStr = new ArrayList<>();
    try{
        for (int i = 0; i < s.length(); i++) {
            subStr.add(s.substring(i, i + k));
        }
        for (int i = s.length(); i == 0; i--) {
            subStr.add(s.substring(i, i - k));
        }
        for (int i = 2; i <= s.length(); i++) {
            subStr.add(s.substring(i, i + k));
        }
    }catch (Exception e){
        e.printStackTrace();
    }

    subStr.sort(null);
    return subStr.get(0) + "\n" + subStr.get(subStr.size() - 1);
}

    public static void main (String[]args){
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        int k = sc.nextInt();

        System.out.println(getSmallestAndLargest(s, k));

        sc.close();

    }