good solution

Didn't know that hashes could be manipulated that way!

Hi, while I was writing the solution I took it for granted but when you asked the question it made me search for why it works, here is a link expalinaing why it works for ruby 1.9+.
https://stackoverflow.com/questions/17355316/hash-ordering-preserved-between-iterations-if-not-modified

I personnaly prefer map/reduce because its how I learned the concept, from big data map/reduce

It never gets added. The way join works is it just joins the elements using the given character, doesn't append after each one.

http://www.ruby-doc.org/core-2.1.2/Array.html#method-i-join

looks good, but I'm not sure I can
read where in the code, the final
comma gets chopped off. Hmmm.

Good for beginners (like me) to learn this.

Doesn't this rely on the order of roman? I thought the whole point of hashes is that the order can be unpredictable...

Can some one put an example to the problem, I don't understand