There are empty values

To my opinion if you would solve the kata in C language you would see that in C it's not simple because of the exact dynamically allocation that you need

Nice

Thanks for the tip, I love free points!

Please stop complaining about kata ranks (not you specifically, just everyone). Changing ranks is expensive, not worth it, and not going to happen. Just enjoy the cheap points.

That shouldn't be 4 kyu. Maybe 7

Description: "A string representation of an integer will contain no characters besides the ten numerals "0" to "9".".

But have this error: System.FormatException : The value could not be parsed.

Because of a is empty. Test 6

Using recursion is allowed, but naive recursive solution is going to fail because of too many recursive calls.
Most of solutions to this kata use recursion, but they use an algorithm which greatly reduces recursion depth.

Test is approved, but the attempt is not: "Stack overflow. Repeat 74808 times". Is using recursion not allowed?

Math.Pow(-1, i) is clever. Forgot about this option