Practice: Reversing Boarding Groups Three at a Time

--- group 1
counted: A B C
list: dummy → C → B → A → D → E → F → G → H → null
before: A
--- group 2
counted: D E F
list: dummy → C → B → A → F → E → D → G → H → null
before: D
--- group 3
counted: G H, fewer than 3
list: dummy → C → B → A → F → E → D → G → H → null
left as it is

class ListNode:
    def __init__(self, val):
        self.val = val
        self.next = None


def build(values):
    head = None
    for val in reversed(values):
        node = ListNode(val)
        node.next = head
        head = node
    return head


def values_of(head):
    values = []
    while head:
        values.append(head.val)
        head = head.next
    return values


def reverse_in_groups(head, k):
    dummy = ListNode("dummy")
    dummy.next = head
    before = dummy
    while True:
        end = before
        for _ in range(k):
            end = end.next
            if end is None:
                return dummy.next
        first, after = before.next, end.next
        prev, curr = after, first
        while curr is not after:
            next_node = curr.next
            curr.next = prev
            prev = curr
            curr = next_node
        before.next = end
        before = first


print("groups of 3:", *values_of(reverse_in_groups(build(list("ABCDEFGH")), 3)))