【劍指】反轉鏈表

設定一個空指針和一個頭指針，當頭指針不爲空時，執行：
1、暫存後繼指針
2、修改後繼指針
3、存儲當前節點
4、訪問下一節點

# 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