public ListNode removeNthFromEnd(ListNode head, int n) {        if (head.next == null)  return null;        List
    
      l = new ArrayList<>();        while (head != null){            l.add(head);            head = head.next;        }        if (n == 1){            ListNode br = l.get(l.size() - 2);            br.next = null;            return l.get(0);            }        else if(n == l.size())            return l.get(1);        else{            ListNode fr = l.get(l.size() - n - 1);  //n == n            ListNode br = l.get(l.size() - n + 1);  //n == 1            fr.next = br;            return l.get(0);            }    }
    

利用快慢指针：

这里快指针要走n+1步，会出现走过界的情况，就在他们前面加入一个start节点，完全不影响，因为slow和fast始终差n.

public ListNode removeNthFromEnd(ListNode head, int n) {        //利用快慢指针        ListNode start = new ListNode(0);        ListNode slow = start, fast = start;        slow.next = head;        //slow,fast 相隔n个,让fast 先走n+1步        for (int i=0; i <= n ; i++)            fast = fast.next;                while (fast != null){            fast = fast.next;            slow = slow.next;        }                slow.next = slow.next.next;        return start.next;    }