Some interview questions

You have 2 files:
a.txt
aaa bbb ccc fddf
ddd eee fff

b.txt
111 222 333
444 555 666

Mix their output like this:
aaa 111 bbb 222 ccc 333
ddd 444 eee 555 fff 666

My ruby solution:

atext=File.readlines('a.txt').map {|line| line.split}

btext=File.readlines('b.txt').map {|line| line.split}

out=[]

(btext.size > atext.size ? btext.size : atext.size).times do |i|

    if btext[i].nil?

        out<<atext[i]

    elsif atext[i].nil?

        out<<btext[i]

    else

        out<<atext[i].zip(btext[i]).flatten.compact

    end

end

File.open("out.txt", "w+") do |f|

  out.each { |element| f.puts(element.join(' '))}

end