queue_with_single_stack.rb

class QueueWithSingleStack

  def initialize
    @stack = []
  end

  def enqueue(item)
    @stack << item
  end

  def dequeue
    if @stack.empty?
      return nil 
    else
      return recurse()
    end
  end

  def recurse
    top = @stack.pop
    if @stack.empty?
      return top
    else
      item = recurse
      @stack << top
      return item
    end
  end
end

queue = QueueWithSingleStack.new
queue.enqueue(1)
queue.enqueue(2)
queue.enqueue(3)
queue.enqueue(4)
p queue.dequeue # => 1
p queue.dequeue # => 2
queue.enqueue(5)
p queue.dequeue # => 3
p queue.dequeue # => 4
p queue.dequeue # => 5