逆ポーランド記法

rpn.rb

def rpn(expression, separator = '')
  stack = []
  expression.split(separator).each do |token|
    if token =~ /[\d.]+/
      stack.push token.to_f
    else
      rhs = stack.pop
      lhs = stack.pop
      stack.push lhs.send(token, rhs)
    end
  end
  stack.pop
end