When discussing numbers, it is important to note the boundary. You said integer, but are negatives possible? is the range outside the 16bit signed limit? 32bit signed limit? is there any limit at all?

When looking for performance solutions, you should also provide information on the platform. Do you need portability? are you on a win32 or unix platform? are you capable of understanding C code? (an Inline::C function would make short work of your problem)

And if possible, providing context is valuable. There have been numerous times when people have posted a very limited description, asking a very targetted question, and it was found that the entire operation could be avoided by use of a different overall strategy. Context helps experienced monks identify those situations.