queue_with_two_stacks.rb

class QueueWithTwoStacks

  def initialize
    @s1 = []
    @s2 = []
  end

  def enqueue(item)
    @s1 << item
  end

  def dequeue
    if @s2.empty?
      while !@s1.empty?
        @s2 << @s1.pop
      end
    end
    @s2.pop
  end

end

queue = QueueWithTwoStacks.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