19

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode RemoveNthFromEnd(ListNode head, int n) {
        var dummy = new ListNode(0, head);
        var slow = dummy;
        var fast = dummy;
 
        for (int i = 0; i < n; i++)
            fast = fast.next;
 
        while (fast.next != null) {
            slow = slow.next;
            fast = fast.next;
        }
 
        slow.next = slow.next.next;
        return dummy.next;
    }
}

Use two pointers: advance the first pointer N steps ahead, then move both together. When the first reaches the end, the second is at the node to remove.