19

/**
 * 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 dummy = new ListNode(0);
        dummy.next = head;
        ListNode p = dummy;
        ListNode q = dummy;
        int i = 0;
        while (i <= n) {
            p = p.next;
            i++;
        }
        while (p != null) {
            p = p.next;
            q = q.next;
        }
        q.next = q.next.next;
        return dummy.next;
    }
}