返回链表中间的node

	public static Node returnHalfNode(Node head){
		Node lastNode=head;
		while(head!=null){
			halfDataStack.add(head);
			lastNode=lastNode.next.next;
			
			if(lastNode==null){
				return head;
			}
			if(lastNode.next==null){
				return head.next;
			}
			head=head.next;
		}
		return head;
	}