从链表的末端开始，删除第n个节点

Given a linked list, remove the nth node from the end of list and return its head.

For example,

   Given linked list: 1->2->3->4->5, and n = 2.

   After removing the second node from the end, the linked list becomes 1->2->3->5.

Note:
Given n will always be valid.
Try to do this in one pass.

def removeNthFromEnd(head,n):
    counter=0
    ptr=head
    lagptr=head
    while ptr!=None:
        ptr=ptr.next
        if counter>n:
            lagptr=lagptr.next
        counter+=1
    if counter-n>0:
        lagptr.next=lagptr.next.next
        return head
    if counter==n:
        return head.next
    else:
        return None