You have a linked list ↴ and want to find the kth to last node.

kth_to_last_node.rb

#Write a method kth_to_last_node() that takes an integer kk and the head_node of a singly-linked list, and returns the kkth to last node in the list.
 
 def kth_to_last_node(k, head)

  if k < 1
    raise ArgumentError, "Impossible to find less than first to last node: #{k}"
  end

  # STEP 1: get the length of the list
  # start at 1, not 0
  # else we'd fail to count the head node!
  list_length = 1
  current_node = head

  # traverse the whole list,
  # counting all the nodes
  while current_node.next
    current_node = current_node.next
    list_length += 1
  end

  # if k is greater than the length of the list, there can't
  # be a kth-to-last node, so we'll return an error!
  if k > list_length
    raise ArgumentError, "k is larger than the length of the linked list: #{k}"
  end

  # STEP 2: walk to the target node
  # calculate how far to go, from the head,
  # to get to the kth to last node
  how_far_to_go = list_length - k

  current_node = head
  (0...how_far_to_go).each do |i|
    current_node = current_node.next
  end

  return current_node
end