right?

It's readablle, but where is the 'Best Practices' in this solution?

سبحان الله

Bro, religion is how to live your life,
so brother liked to benefit those people, instead of writing a weird #define directives.

جزاك الله كل خير وتوفيق، ورحم الله والديك أخ هيثم الكريم.

There's no need for the last two headers.

But why your solutions passes if the problem says to remove 1 if there is 13th floor, this solution check if f > 13 instead of f >= 13, why?

Cool, thanks! You remembered me of these ways of adding and subtracting boolean results :)

std::islower is not exactly included in the string header :)

Is that required? (i.e. is it important to be included exactly?)

std::islower is not exactly included in the string header :)
Unless there is some other overload, one of them is defined in cctype, and another one in locale.

std::islower is included in the string header.
It's easier to read and understand than writing it by hand.

I don't think recursion is good, anyways :D

You should call it product (the result of multiplication), instead of sum (the result of summation/addition).

I really don't know why 23 one vote to this as best practices.
He even used math.h instead of cmath :)