题解 | #链表内指定区间反转#

# class ListNode:
#     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:
        if m == n:
            return head
        tmp_head = ListNode(-1) # 添加一个头，防止m=1，最后去掉
        tmp_head.next = head
        break_point = tmp_head
        for i in range(m-1):
            break_point = break_point.next  # 断点
        new_tail = break_point.next # 反转区间的新尾部
        cur = new_tail # 指针、
        per = cur.next
        for i in range(n-m):   # 需要n-m次反转
            tmp = per.next
            per.next = cur
            cur = per
            per = tmp
        new_tail.next = per
        break_point.next = cur
        head = tmp_head.next
        return head