stack_with_two_queues.rb

class StackWithTwoQueues

  def initialize
    @q1 = []
    @q2 = []
  end

  def push(item)
    @q1 << item
  end

  def pop()
    while true
      head = @q1.shift
      break if @q1.empty?
      @q2 << head
    end
    
    tmp = @q1
    @q1 = @q2
    @q2 = tmp
    head
  end

end

stack = StackWithTwoQueues.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