from structures import build_list

def cycle_entry(head):
    slow = fast = head
    while fast is not None and fast.next is not None:
        slow = slow.next
        fast = fast.next.next
        if slow is fast:
            break
    else:
        return None
    entry = head
    while entry is not slow:
        entry = entry.next
        slow = slow.next
    return entry