class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        //辅助节点
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        //链表长度
        int len = 0;
        ListNode cur = head;
        while(cur != null){
            ++len;
            cur = cur.next;
        }
        //短了
        if(n > len) return head;
        //移动要删除节点的前一个节点
        n = len - n;
        cur = dummy;
        while(n > 0) {cur = cur.next;--n;}
        //删除
        cur.next = cur.next.next;
        return dummy.next;
    }
}