删除链表倒数第n节点

class :
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        if not head:
            return head
        dummpy = ListNode(0)
        dummpy.next = head
        l = dummpy
        r = dummpy
        count = 1
        while count <= n:
            r = r.next
            count += 1
        while r.next:
            l = l.next
            r = r.next
        l.next = l.next.next
        return dummpy.next