This is a 8 kyu kata so it is simple and has no restrictions like disabling Linq.
Some impovements has been made.
I don't know exacly what You want me to change. You mean kata description or tests or both?
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It's a part of the task.
It's a basic functional programming kata (8 kyu) - leave it as it is please.
Ok. Tests are mismatched. Actual and expected are wrong position but i they still must be the same. My solution works. I can't publish fixed tests (codewars problems).
Tests works fine. Look at the other solutions.
It's up to Youm, what You want to return i, is think null is fine becouse it works easier with comparison operators.