92. Reverse Linked List II（python）

Reverse a linked list from position m to n. Do it in-place and in one-pass.
For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,
return 1->4->3->2->5->NULL.
Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.
题意：翻转链表m到n。
Runtime: 42 ms
思路：参考206翻转整个链表的思路二，可以m到n视为一个链表，依次将结点放在n后续链表的头结点
注意，指针pm指向m前一结点不变，保证移动的结点始终在pm.next，指针pn指向n结点，保证移动的结点插入的位置始终在n后的头结点
循环次数为n-m次

# 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):
        if m==n or not head:
            return head
        dummy=ListNode(0)
        dummy.next=head
        pm=pn=dummy
        for i in range(m-1):
            pm=pm.next
        for i in range(n):
            pn=pn.next
        for i in range(n-m):
            t=pm.next
            pm.next=pm.next.next
            t.next=pn.next
            pn.next=t
        return dummy.next