19. Remove Nth Node From End of List

/**
 * 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(42);
        dummy.next = head;

        // example: n = 2
        // dummy ->  1  ->  2  ->  3  ->  4  ->  5  ->  6  ->
        //  slow
        //  fast
        ListNode slow = dummy;
        ListNode fast = dummy;

        // dummy ->  1  ->  2  ->  3  ->  4  ->  5  ->  6  ->
        //  slow
        //                 fast
        for (int i = 0; i < n; i++) {
            fast = fast.next;
        }

        // dummy ->  1  ->  2  ->  3  ->  4  ->  5  ->  6  ->
        //                               slow
        //                                             fast
        while (fast.next != null) {
            slow = slow.next;
            fast = fast.next;
        }

        // dummy ->  1  ->  2  ->  3  ->  4  ->  6  ->
        //                               slow
        //                                      fast
        ListNode removed = slow.next;
        slow.next = slow.next.next;
        removed.next = null;

        return dummy.next;
    }
}