反转链表

思路：

 

 一开始需要将pre的next置空

代码：

 

 public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;
        while (cur != null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }


        return pre;
    }