Add a Node to the End

addFirst() is already written for you. Appending is the expensive twin: with no tail pointer you must walk the whole chain to reach the last node, so addLast() is O(n).

Requirements:
1. addLast(int value) appends a node holding value to the back of the list.
2. When the list is empty the new node becomes head.
3. Otherwise walk to the node whose next is null and link the new node there.

The Node class is provided - do not rename it and do not add public to it, because a Java source file may contain only one public class.

Do NOT write a main method.