public static String getSmallestAndLargest(String s, int k) {

            String smallest = "";
        String largest = "";
    java.util.List<String> arr = new java.util.ArrayList<>();

    int l = s.length() - k;

    for (int i = 0; i <= l; i++) {
        String c = s.substring(i, i+k);
        arr.add(c);
    }
    java.util.Collections.sort(arr);
    smallest = arr.get(0);
    largest = arr.get(arr.size() - 1);


    // Complete the function
    // 'smallest' must be the lexicographically smallest substring of length 'k'
    // 'largest' must be the lexicographically largest substring of length 'k'

    return smallest + "\n" + largest;
}