I can not diecide between BP & C

This is a task that, like all tasks here, requires you to provide a valid solution, whatever it is. If an author doesn't want some library to be used, it's up to him to make it impossible.

I beleave This is not a task for using standard library from STL. I thnk they wanted us to create our own logic.

It's a puzzle, actually a puzzle that may serve you in a certification exam. Point what you think that could be clarified. BTW you already got the hint unicode ;)

Some clarification on how the degree of accuracy required. Either that or relax the testsuite to only check accuracy for 7 or 8 digits

Clearer instructions