一、25. K 个一组翻转链表
1.1、206. 反转链表

Python 代码实现
class ListNode:
def __init__(self, val=0, next=None):
self.val = val
self.next = next
class Solution:
def reverseList(self, head: ListNode) -> ListNode:
pre = None
cur = head
while cur:
nxt = cur.next
cur.next = pre
pre = cur
cur = nxt
return pre
1.2、92. 反转链表 II

思路
整体来看,p0.next 是反转前的第 left 个节点,pre 是反转后的头节点,cur 是当前遍历到的节点。
Python 代码实现
class Solution:
def reverseBetween(self, head: ListNode, left: int, right: ) -> ListNode:
p0 = dummy = ListNode(=head)
_ (left - ):
p0 = p0.
pre =
cur = p0.
_ (right - left + ):
nxt = cur.
cur. = pre
pre = cur
cur = nxt
p0.. = cur
p0. = pre
dummy.






