linked_list.rb

# LinkedList implementation as given in the problem assignment
class LinkedList
  attr_accessor :payload, :next_element

  def initialize(payload, next_element)
    @payload, @next_element = payload, next_element
  end
end

# output_list implementation as given in the problem assignment
def output_list(list)
  return if list.nil?
  puts list.payload
  output_list(list.next_element)
end

stack.rb

require "./linked_list"

# Stack implementation as given in problem assignment
class Stack
  attr_reader :data

  def initialize
    @data = nil
  end

  # Push an item onto the stack
  def push(element)
    @data = LinkedList.new(element, @data)
  end

  # Pop an item off the stack.
  # Remove the last item that was pushed onto the stack
  # and return it to the user
  def pop
    unless @data.nil?
      element = @data.payload
      # advance pointer
      @data = @data.next_element
      # return element
      element
    end
  end
end

stack_test.rb

require 'minitest/autorun'
require './stack'

describe Stack do
  before do
    @stack = Stack.new
  end

  describe 'when empty' do
    it "pop doesn't return anything" do
      @stack.pop.must_equal nil
    end
  end

  describe 'adding elements' do
    it 'returns elements in reverse order of adding' do
      @stack.push 1
      @stack.push 2
      @stack.push 3
      @stack.pop.must_equal 3
      @stack.pop.must_equal 2
      @stack.pop.must_equal 1
      @stack.pop.must_equal nil
    end
  end

end