This kata is about singly 
linked list.  A linked list is an ordered set of data elements, each containing a link to its successor (and sometimes its predecessor, known as a double linked list).  You are you to implement an algorithm to find the kth to last element.
For example given a linked list of:
a -> b -> c -> d
if k is the number one then d should be returned
if k is the number two then c should be returned
if k is the number three then b should be returned
if k is the number four then a should be returned
if k exceeds the size of the list then None returned
Special Note --> Node classes contain two fields; data and next.
And to access the head of the list, use head. e.g. linked_list.head
def search_k_from_end(ll, k):
    def to_list(n):
      return [n.data] if not n.next else [n.data] + to_list(n.next)
    l = list(reversed(to_list(ll.head)))
    return l[k-1] if k<=len(l) else None
def search_k_from_end(ll, k):
    def to_list(n):
      return [n.data] if not n.next else [n.data] + to_list(n.next)
    l = list(reversed(to_list(ll.head)))
    return l[k-1] if k<=len(l) else None