Sure, and one such constraint is to make a solution in as few bytes as possible :-)

I find it very hard not to use features of the particular programming language. Clearly, that is the whole point of having different programming languages. Take JavaScript you can 'exploit' the default functions/methods - or you can build your solution using lambda calculus only (disregarding the language features of functions being first class citizens). It's all your choice. However, translating the top-most solution into native C using no libraries shouldn't be too hard as the procedure to solve the kata will most likely go in the same direction.