19. Remove Nth Node From End of List

Remove the nth node from the end of a linked list and return its head.

Move fast n + 1 steps ahead of slow (both starting at a dummy). Then move both until fast falls off the end; slow is right before the target.

1function removeNthFromEnd(head: ListNode | null, n: number): ListNode | null {
2  const dummy = new ListNode(0, head);
3  let slow: ListNode = dummy, fast: ListNode | null = dummy;
4  for (let i = 0; i <= n; i++) fast = fast!.next;
5  while (fast) { slow = slow.next!; fast = fast.next; }
6  slow.next = slow.next!.next;
7  return dummy.next;
8}

list

slow,fast

→

→ ∅

Both pointers start at the dummy.

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Space: play/pause · ←/→: step