It's by design. Personally, I like to figure out things as I go along, which is why I added the "figure out the rest from the given test cases" part to the description. (It also is more like "real" work. You seldom have perfect requirements to start with.)

There are some hints, though. If you arrange the strings as in the example, you'd see that order is important. Also, without that requirement, it wouldn't be a 5 kyu kata.

I've gotten feedback both ways if this is a good approach or not, but I've decided to leave the kata as it is right now.

Hope you enjoyed it anyway. :-)

Thanks for your feedback!

The requirements are not realy clear:

"The restriction is that the characters in part1 and part2 are in the same order as in s."

And then there is a test:

canHandleCharactersInWrongOrder(StringMergerTest)