Code:
the numbers however are almost unconstrained in length

Jooel:

I think that should answer your question. However, I can assure you that I do not have a test that e.g. consists of a string with e.g. 20.000 numeric characters. Though, the solution I came up with would be absolutely blind to such occurrances, anyway.

Not sure what you guys are coding up but the approach I found is actually hinted at in the problem description itself and would not really care about numbers and alpha characters. You could also use all kinds of sorts but a single simple bubblesort would work, too \:\)
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There are two types of vessels, submarines and targets.