台上有三扇门，其中一扇门后藏有现金100万，主持人叫你选择其中一扇门后，然后主持人在余下的门中排除一个没有现金的门，这时问你要不要更换选择？

door_select.rb

require 'pp'
class Door

  # name: :A, :B, :C
  # has_money: true / false
  attr_accessor :name, :has_money

  def initialize(name)
    @name = name
    @has_money = false
  end
end

class Person
  # selected_door :A, :B, :C
  attr_accessor :selected_door
end

class TestCase1

  attr_accessor :doors, :person

  def initialize
    @doors = [Door.new(:A), Door.new(:B), Door.new(:C)]
    @doors[rand(3)].has_money = true
    @person = Person.new
    @person.selected_door = [:A, :B, :C][rand(3)]
  end

  def success?
    @person.selected_door == @doors.detect { |door| door.has_money == true }.name
  end

end

class TestCase2

  attr_accessor :doors, :person

  def initialize
    @doors = [Door.new(:A), Door.new(:B), Door.new(:C)]
    @doors[rand(3)].has_money = true
    @person = Person.new
    @person.selected_door = [:A, :B, :C][rand(3)]
    # puts "First times:choose #{@person.selected_door}"
    change_to_another_door
  end

  def change_to_another_door
    leave_doors = @doors.select { |door| door.name != @person.selected_door }
    #puts "left doors #{leave_doors.collect{|d| d.name }.join(', ')}"
    if success?
      @person.selected_door = leave_doors.collect { |door| door.name }[rand(2)] # will always choose wrong one
    else
      @person.selected_door = leave_doors.detect { |door| door.has_money == true }.name # will always choose right one
    end
  end

  def success?
    @person.selected_door == @doors.detect { |door| door.has_money == true }.name
  end


end

n = 10000

test1_cases = []
n.times do |i|
  test1_cases << TestCase1.new
end

success_cases = test1_cases.select { |t| t.success? }
puts "1. sucess cases count #{success_cases.size}/#{n} [#{success_cases.size/n.to_f}] "

test2_cases = []
n.times do |i|
  test2_cases << TestCase2.new
end

success_cases = test2_cases.select { |t| t.success? }
puts "2. sucess cases count #{success_cases.size}/#{n} [#{success_cases.size/n.to_f}]"