rpn-lesson24.rb

require 'stack'

class RPN
  def initialize
    @stack = Stack.new
  end

  def calc(expr)
    expr.split.each do |e|
      if %w[ + - * / ].include? e
        i = @stack.pop
        @stack.push @stack.pop.send(e, i)
      else
        @stack.push e.to_i
      end
    end
    @stack.pop
  end
end