Ruby Linked List implementation
Looks pretty good to me. You don't need the explicit return as the last statement of your lookup method. Also, in newer versions of Ruby (2.3+ I believe), you don't need the explicit nil check--instead, you can use the Safe Navigation Operator (&amp;.).

  def lookup(lookup_value)
    return self if value.eql? lookup_value
    self.next&amp;.lookup(lookup_value)
  end