学习资料：https://programmercarl.com/0024.两两交换链表中的节点.html
学习记录：
24.两两交换链表中的节点（添加虚拟头节点；交换1、2节点和3、4节点时，要用1前面的cur，先保存1为temp且3保存为temp1，cur指向2，再把2指向temp，因为cur指向2后就与1没关联了）
点击查看代码
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
dummy_head = ListNode(next=head)
cur=dummy_head
while cur.next and cur.next.next:
temp=cur.next
temp1=cur.next.next.next
cur.next=cur.next.next
cur.next.next=temp
temp.next=temp1
cur=cur.next.next
return dummy_head.next
19.删除链表的倒数第N个节点（用快慢指针，fast和slow都设置为虚拟头节点，fast先走N+1步，之后slow和fast同时走，当fast走到NULL，此时slow为倒数第N-1个节点；把slow的下一个节点删除即可）
点击查看代码
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
dummy_head=ListNode(next=head)
slow=fast=dummy_head
for i in range(n+1):
fast=fast.next
while fast:
fast=fast.next
slow=slow.next
slow.next=slow.next.next
return dummy_head.next
点击查看代码
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
fast=slow=head
while fast and fast.next:
fast = fast.next.next
slow = slow.next
if fast == slow:
index1 = fast
index2 = head
while index1 != index2:
index1 = index1.next
index2 = index2.next
return index1
return None
PS: ● 面试题 02.07. 链表相交 还没做，放一下
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