head-->  ["foxtrot",-]--> ["echo", -]--> ["foo",-] -->  ["zebra",null]                 
                                               ^
                                              cur

insertAtTail( "zebra" );

1st step: check for empty list: i,e see if the number of elements in list is 0
	simplest test:  is head null?
        What do we do it if is? we do insertAtFront and immed go home

2nd step  ITS NOT empty so get a pointer at the last node 	          
      	Node cur = head;
	while ( the node curr points to does not have null next field  )
		cur = cur.next; 

step 3:  we have a pointer to the last node 
         curr.next =  new Node ( zebra null )