Hackerank Ruby Super Stack

superstack.rb

operations = ['push 4', 'push 5', 'inc 2 1', 'pop', 'pop']

def superStack(operations)
  stack = []
  size = 0
  operations.each do |operation|
    parameters = operation.split

    if parameters[0] == "push" then
      stack[size] = parameters[1].to_i
      size+=1
    elsif parameters[0] == "pop" then
      stack.pop
      size-=1
    elsif parameters[0] == "inc" then
      # need optimization
      (0..parameters[1].to_i-1).each do |i|
        stack[i] += parameters[2].to_i
      end
    end

    if size == 0 then
      puts "EMPTY"
    else
      puts stack[size - 1]
    end
  end
end

superStack(operations)