class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        
        ListNode start = new ListNode(0);

        // two pointer
        ListNode slow = start, fast = start;
        start.next = head;
        
        
        for (int i = 1; i <= n + 1; i++) {
            fast = fast.next;
        }
        
        while (fast != null) {
            slow = slow.next;
            fast = fast.next;
        }

        slow.next = slow.next.next;

        return start.next;
    }
}
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
/**
[Question]
- remove the nth node from the end of the list
- return its head.

[Condition]
- The number of nodes in the list is sz. 
**/