public static String getSmallestAndLargest(String s, int k) {
        String smallest = "";
        String largest = "";
        smallest = "z";
        for(int i = 0, j = k; j <= s.length(); i++, j++){
            String str = s.substring(i,j);
            if(str.compareTo(smallest) < 0){
                smallest = str;
            }if(str.compareTo(largest) > 0 ){
                largest = str;
            }
        }