19. Remove Nth Node From End of List

public ListNode removeNthFromEnd(ListNode head, int n) {
        if (head.next == null)  return null;
        List<ListNode> 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;
    }