【剑指】反转链表

设定一个空指针和一个头指针，当头指针不为空时，执行：
1、暂存后继指针
2、修改后继指针
3、存储当前节点
4、访问下一节点

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        p,pre = head, None
        while p:
            tmp = p.next
            p.next = pre
            pre = p
            p = tmp
        return pre