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

    String[] arr = new String[s.length()+1-k];
    for(int i =0 ; i<s.length();i++){
        if(i!=s.length()-k){
            arr[i] = s.substring(i, k+i);
        }else{
            arr[i] = s.substring(i, k+i);
            break;
        }
    }
    smallest = arr[0];
    largest = arr[0];

    for (int i=1;i< arr.length;i++){
        if(arr[i].compareTo(largest)>0){
            largest = arr[i];
        }
        if (arr[i].compareTo(smallest)<0) {
            smallest= arr[i];
        }
    }
    return smallest + "\n" + largest;
}