gistfile1.txt

def calculate(s)
  stack = []
  num = 0
  op = '+'
  
  s.each_char.with_index do |char, i|
    if char.between?('0', '9')
      num = num * 10 + char.to_i
    end
    
    if "+-*/".include?(char) || i == s.length - 1
      case op
      when '+'
        stack.push(num)
      when '-'
        stack.push(-num)
      when '*'
        stack.push(stack.pop * num)
      when '/'
        last = stack.pop
        last = last > 0 ? last / num : -(-last / num)
        stack.push(last)
      end
      
      op = char
      num = 0
    end
  end
  
  stack.sum
end