This is concise and nice to read, but becomes very slow for large arrays. Taking (1..1000000).to_a as an input, took nearly 5 min on my (old) machine.

No problem at all! But thanks for the note (and for the kata :-). I've submitted a modified solution, using the new method name.

I'll keep that in mind, thanks a lot!

Oh, I see. I tried a recursive solution and didn't think about keeping the input array intact. Thanks for the pointer :-)

I might be wrong, but it seems like the random tests (for Ruby) always expect the value of x as the return value.

Random tests
Testing for 12 and [712, 6636, 959, 81, 3734, 70, 1172, 33, 6, 561, 237, 749, 27, 74, 1, 7370, 8, 7335, 53, 5323]
It should work for random inputs too - Expected: 12, instead got: 143

Yes, was talking about Ruby. Sorry for not mentioning the language.

Works like a charm now. Thanks for the quick fix!