You're mutating the input list. That was the reason why it failed. I've approved the fork below so tampering with it won't be a problem anymore.

I agree this should be explained in the kata better. Knowing this helped me solve this much faster.

'112123123412345123456123456712345678123456789123456789101234567891011123456789101112123456789101112131234567891011121314'[100-1] (1 based) => 1 not 9

Please see description and example test cases.

for n = 100, there are only 3 numbers that meet the requirement. They are 5, 55, 77.

for n = 1000, there are only 11 numbers.

Maybe some trailing space? Read this and if that doesn't help, try posting your code following the directions there so we can see your code.

That's part of the testing suite, it can't be removed.

Expected: equal to Jacob and Alex like this
                   ************************
Actual: Jacob and Alex likes this
        *************************

You should read it like that