给你一个链表，删除链表的倒数第 n 个结点，并且返回链表的头结点

    public ListNode removeNthFromEnd(ListNode head, int n) {
         ListNode before = head;
         ListNode after = head;
         if(head==null){
             return null;
         }
         while(n!=0){
             before = before.next;
             n--;
         }
         if(before==null){//删除头结点
             return head.next;
         }
         while(before.next!=null){
             before = before.next;
             after = after.next;
         }
         after.next = after.next.next;//删除中间结点
         return head;
    }