reverse a linked list, k nodes at a time.

reverse_list.d

Node reverse(Node head, int k) {
	return reverse(head, k, length(head));
}

Node reverse(Node head, int k, int remaining) {
	if (remaining < k) return head;

	auto i = 0;
	Node next, prev = null, first = head;
	while (head && i < k) {
		next = head.next;
		head.next = prev;
		prev = head;
		head = next;
		++i;
	}
	first.next = reverse(head, k, remaining - k);
	return prev;
}