19. Remove Nth Node From End of List|Time: O(n)|Space: O(1)

// Problem: https://leetcode.com/problems/remove-nth-node-from-end-of-list
// Solution: https://leetcode.com/problems/remove-nth-node-from-end-of-list/solution/

/**
 * 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 first = dummy;
        ListNode second = dummy;

        // Advances first pointer so that the gap between first and second is n nodes apart
        for (int i = 1; i <= n + 1; i++) {
            first = first.next;
        }

        // Move first to the end, maintaining the gap
        while (first != null) {
            first = first.next;
            second = second.next;
        }

        second.next = second.next.next;
        return dummy.next;
    }
}