虛擬頭結點的作用是：簡化插入/刪除邏輯+方便返回頭節點+減少邊界錯誤

Leetcode206鏈表反轉

206. 反轉鏈表 - 力扣（LeetCode）

頭插法

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):def reverseList(self, head):""":type head: Optional[ListNode]:rtype: Optional[ListNode]"""#定義current，dumpyheaddumpyhead=ListNode(-1)dumpyhead.next=None#因為希望當前節點指向的next是dumpy的nextcurrent=headwhile current!=None:tmp=current.next#記憶當前結點下一節點是什么current.next=dumpyhead.nextdumpyhead.next=currentcurrent=tmpreturn dumpyhead.next

Leetcode6刪除鏈表的倒數第N個結點

19. 刪除鏈表的倒數第 N 個結點 - 力扣（LeetCode）

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):def removeNthFromEnd(self, head, n):""":type head: Optional[ListNode]:type n: int:rtype: Optional[ListNode]"""#不能先初始化在head，否則因為n+1可能超出# slow=head# fast=headdummynode=ListNode(-1,head)slow=dummynodefast=dummynodefor i in range(n+1):fast=fast.nextwhile(fast):fast=fast.nextslow=slow.nextslow.next=slow.next.nextreturn dummynode.next