python语法糖，不用白不用

    def ReverseList(self , head: ListNode) -> ListNode:
        if not head:	# 如果非空
            return head
        a, a.next, b =head, None, head.next		# 初始化
        while b:
            b, a, a.next = b.next, b, a			# 遍历
        return a