@wenima: The description of the task is to return "a matching list in the reverse order.", not to reverse the list in-place.

It's OK now. Thanks!

I think the C# random tests may be broken. For example, for 00 + 0011 I get the error:

✘     Expected string length 1 but was 2. Strings differ at index 0.
Expected: "0"
But was:  "11"

I just completed this exercise in Ruby and the tests seem to be OK.