detect_cycle_start.py

def get_pointers_meetingPoint(head):
    slow = head
    fast = head

    while fast and fast.next:

        slow = slow.next
        fast = fast.next.next

        if slow == fast:
            return slow

    return None


def get_cycle_start(head, meeting_point):

    p = head
    q = meeting_point

    while p != q:
        p = p.next
        q = q.next

    return p


if __name__ == '__main__':

    meeting_point = get_pointers_meetingPoint(head)

    if not meeting_point:
        return None
    else:

        return get_cycle_start(head, meeting_point)