Nice

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

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

Nice Kata

The return case is not considered fully. When there is no missing letters, what should I return? In my solution it accepted: return 'x'; But I think this is not correct to return x character, when there is not missing letters. You might think that x is actually missing when its not

Can you explain that logic inside foreach?

Glad to see not only me used Backwards Combination.

Tested this solution and mine with BenchmarkDotNet.

• The mean is:

Linq: 878.89 ns

Without: 44.05 ns

• Error:

Linq: 5.062 ns

Without: 0.266 ns

I thought about this approach too, but tried another. Then tested with linq and my and surprisingly linq is 4-5 times faster.

This logic is much more concise than mine. I like it. Mine is a bit overcomplicated =(

Intresting linq variation

You can't use out as a variable name