Các nút đảo ngược trong nhóm K

def reverse_k_group_opt(head, k: int):
    if isinstance(head, list):
        h = build_linked_list(head)
        res = reverse_k_group_opt_helper(h, k)
        return linked_list_to_list(res)
    return reverse_k_group_opt_helper(head, k)

def reverse_k_group_opt_helper(head: ListNode, k: int) -> ListNode:
    curr = head
    for _ in range(k):
        if not curr:
            return head
        curr = curr.next
    prev = None
    curr = head
    for _ in range(k):
        nxt = curr.next
        curr.next = prev
        prev = curr
        curr = nxt
    head.next = reverse_k_group_opt_helper(curr, k)
    return prev

def reverse_k_group_brute(head, k: int):
    if isinstance(head, list):
        h = build_linked_list(head)
        res = reverse_k_group_brute_helper(h, k)
        return linked_list_to_list(res)
    return reverse_k_group_brute_helper(head, k)

def reverse_k_group_brute_helper(head: ListNode, k: int) -> ListNode:
    # Recursively reverse k elements by extracting values
    curr = head
    vals = []
    for _ in range(k):
        if not curr: return head
        vals.append(curr.val)
        curr = curr.next
    curr = head
    for v in reversed(vals):
        curr.val = v
        curr = curr.next
    reverse_k_group_brute_helper(curr, k)
    return head