反转链表

思路：cur.next = pre

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

# @param head ListNode类 
# @return ListNode类
class Solution:
    def ReverseList(self , head: ListNode) -> ListNode:
        # write code here
        # 空链表处理
        if head == None:
            return head
        # 非空链表处理
        pre = None
        cur = head
        while cur:
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
        return pre