one pass

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode fast = head;
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        n--;
        while (n > 0){
            fast = fast.next;
            n--;
        }

        ListNode slow = dummy;
        while (fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next;
        }
        slow.next = slow.next.next;

        return dummy.next;
    }
}