public static String getSmallestAndLargest(String s, int k) {
        String smallest = "";
        String largest = "";
        
        String[] subStr = new String[s.length() - (k-1)];
        subStr[0] = s.substring(0,k);
        
        // adding all substrings to a array 
        for(int i = 1; i < subStr.length; i++) {
        	subStr[i] = s.substring(i,i + k);
        }
        
        // using bubble sort for lexicographically element order
        for(int i = 0; i < subStr.length; i++) {
        	for(int j = 0; j < subStr.length - i - 1; j++) {
        		if(subStr[j].compareTo(subStr[j + 1]) > 0) {
        			String temp = subStr[j];
        			subStr[j] = subStr[j + 1];
        			subStr[j + 1] = temp;
        		}
        	}
        }
        
        smallest = subStr[0];
        largest = subStr[subStr.length - 1];
        
        return smallest + "\n" + largest;
    }