```
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def reverseBetween(self, head, m, n):
"""
:type head: ListNode
:type m: int
:type n: int
:rtype: ListNode
"""
dummy = ListNode(0)
dummy.next = head
cur = dummy
for i in range(1, m):
cur = cur.next
tail = cur.next
for i in range(m, n):
if tail is None:
break
tmp = tail.next
tail.next = tmp.next
tmp.next = cur.next
cur.next = tmp
return dummy.next
```