题解 | #链表内指定区间反转#

# class ListNode:

# from numpy._typing import _NBitLongDouble
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 
# @param m int整型 
# @param n int整型 
# @return ListNode类
#
class Solution:
    def reverseBetween(self , head: ListNode, m: int, n: int) -> ListNode:
        # write code here
        if not head:
            return 
        # 头插法虚拟节点
        dummy_head = ListNode(0) 
        # 头插法
        count = 1
        temp = dummy_head  #这里主要是记录变化
        temp2 = ListNode(0)  # 这个用来记录倒序的开始节点
        # 主要想法是： 把head逐个节点复制下来，当需要倒叙的时候（如题目中链条2-3-4），记录下当前倒叙节点（如temp2=2）；把需要倒叙的链条转换（转换后4-3-2）;转换后的最后节点为已经记录的temp2，所以从此之后继续逐个复制。
        while head:
            if count >= m and count <=n:
                # 提前记录
                tmp = head  
                head = head.next
                # 将当前节点在头部插入
                tmp.next = temp.next
                temp.next = tmp
                if count == m:
                    temp2 = tmp
 
                if count == n:
                    temp = temp2
                    # for i in range(n-m+1):
                    #     temp = temp.next
            else:
                new_node = ListNode(head.val)
                temp.next = new_node
                temp = new_node
                head = head.next
                # temp = temp.next
     
            count = count + 1

        # dummy_head = temp
        return dummy_head.next