stack_with_single_queue.rb

class StackWithSingleQueue

  def initialize
    @queue = []
  end

  def push(item)
    @queue << item
  end

  def pop()
    size = @queue.size
    (size-1).times do
      @queue << @queue.shift
    end
    @queue.shift
  end

end

stack = StackWithSingleQueue.new
stack.push(1)
stack.push(2)
stack.push(3)
stack.push(4)
p stack.pop() # => 4
p stack.pop() # => 3
stack.push(5)
p stack.pop() # => 5