My solution to the Reverse Polish Notation Problem

RPNCalculator.rb

class RPNCalculator
  def evaluate(rpn_string)
    rpn_arr = rpn_string.split(' ')
    operating_arr = []

    rpn_arr.each do |element|
      operating_arr << case element
        when /\d+/ then element.to_i
        when '+' then operating_arr.pop + operating_arr.pop
        when '*' then operating_arr.pop * operating_arr.pop
        when '-' then - operating_arr.pop + operating_arr.pop
      end
    end
    operating_arr.last
  end
end


calc = RPNCalculator.new

p calc.evaluate('1 2 +')   # => 3
p calc.evaluate('2 5 *')   # => 10
p calc.evaluate('50 20 -') # => 30

# The general rule is that 'A B op' is the same as 'A op B'
# i.e., 5 4 - is 5 - 4.

p calc.evaluate('70 10 4 + 5 * -') # => 0