定义一个函数，输入一个链表的头节点，反转该链表并输出反转后链表的头节点。

    示例:

    输入: 1->2->3->4->5->NULL
    输出: 5->4->3->2->1->NULL

    限制：

    0 <= 节点个数 <= 5000

    1. class Solution:
    2.     def reverseList(self, head: ListNode) -> ListNode:
    3.         if head is None:
    4.             return head
    5.         newhead = None
    6.         cur = head
    7.         nextnode = head.next
    8.         while nextnode != None:
    9.             cur.next = newhead
    10.             newhead = cur
    11.             cur = nextnode
    12.             nextnode = nextnode.next
    13.         cur.next = newhead
    14.         newhead = cur
    15.         return newhead