class Solution:

    def reverseBetween(self, head: ListNode, m: int, n: int) -> ListNode:

        dummy = ListNode(0)

        dummy.next = head

        pre = dummy

        for i in range(m-1):

            pre = pre.next

        start = pre.next

        trail = start.next

        for i in range(n-m):

            start.next = trail.next

            trail.next = pre.next

            pre.next = trail

            trail = start.next

        return dummy.next

# Definition for singly-linked list.

# class ListNode:

#     def __init__(self, x):

#         self.val = x

#         self.next = None

class Solution:

    def reverseBetween(self, head: ListNode, m: int, n: int) -> ListNode:

        dummy = ListNode(0)

        dummy.next = head

        pre = dummy

        for i in range(m-1):

            pre = pre.next

        node =None

        cur = pre.next

        for i in range(n-m+1):

            tmp = cur.next

            cur.next = node

            node = cur

            cur = tmp

        pre.next.next = cur

        pre.next = node

        return dummy.next