class StackTest < Test::Unit::TestCase
  test "new stack should have 0 elements" do
    assert_equal nil, Stack.new.size
  end

  test "new stack tail should be nil" do
    assert_equal nil, Stack.new.tail
  end

  test "after push, stack should have one element" do
    stack = Stack.new
    stack.push 1
    assert_equal 1, stack.size
  end

  test "after pushing 'foo', stack pop should return 'foo'" do
    stack = Stack.new
    stack.push 'foo'
    assert_equal 'foo', stack.pop
  end
end

class Stack
  def initialize
    @items = []
    @size = 0
  end

  def size
    @size
  end

  def push(item)
    @items << item
    @size += 1
  end

  def tail
    @size == 0 ? nil : @items[@size - 1]
  end

  def pop
    raise RuntimeError if @size == 0
    @items[@size - 1] = nil
    @size -= 1
  end
end