反转列表-递归

 public ListNode reverseList(ListNode head) {
 	//返回原链表的尾节点，每次递归都返回这个节点
	if (head == null || head.next == null) {
	    return head;
	}
	ListNode p = reverseList(head.next);
	//形成环
	head.next.next = head;
	//断开环
	head.next = null;
	return p;
}