Solution.java

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode first = dummy;
        ListNode second = dummy;
        
        // Advance first pointer so that the gap between first and second is n nodes apart
        for (int i = 0; i <= n; i++) {
            first = first.next;
        }
        
        // Move first to the end, maintaining the gap
        while (first != null) {
            first = first.next;
            second = second.next;
        }
        
        // Remove the nth node from the end
        second.next = second.next.next;
        return dummy.next;
    }
}