Yes I know. But test should be able to catch that, right? I already commented this in the discussion. I also did rework my solution :)

This shouldn't work for huge numbers. Unfortunatelly there's no test written for that, but imagine 1000 digit number. There's a reason why is it passed as std::string.

You should add test with a large number that is divisible by 11. I used long long and simple stringstream and everything passed. I guess there is no test covering that scenario and my solution just happen to pass.