链表---给定一个链表，删除链表中倒数第n个节点，返回链表的头节点

链表中的节点个数大于等于n

给出链表1->2->3->4->5->null和 n = 2.

删除倒数第二个节点之后，这个链表将变成1->2->3->5->null

 ListNode removeNthFromEnd(ListNode head, int n) {
        // write your code here
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode fast = dummy;
        ListNode slow = dummy;
        while (fast != null && n > -1) {
            fast = fast.next;
            n--;
        }
        while (fast != null) {
            fast = fast.next;
            slow = slow.next;
        }
        slow.next = slow.next.next;
        return dummy.next;
    }