92. 反转链表 II

class Solution:
    def reverseBetween(self, head, m, n):
        """
        :type head: ListNode
        :type m: int
        :type n: int
        :rtype: ListNode
        """
        if head == None or head.next == None:
            return head
        h = ListNode(-1)
        h.next = head
        pre = h
        cur = head
        i = 1
        while i < m:
            pre = cur
            cur = cur.next
            i += 1
        t1 = pre
        t2 = cur
        while i <= n:
            lat = cur.next
            cur.next = pre
            pre = cur 
            cur = lat
            i += 1
        t1.next = pre
        t2.next = cur
        return h.next