this is my meaning, wy you allow such a crap? using Biginteger should be forbidden or it is 8kyu and Not 4..
you should read first, before answer

nice solution and one clever for not using lame bigInteger

you can do it with string and byte only... one digit needs one byte. Now you can do it like in school

what would you do if lame biginteger isn't avalable? the answer is easy as pie. like in school calc it digit by digit.
Here my solution :-)

https://www.codewars.com/kata/reviews/553a8610f3cc94c5e40000cb/groups/5f5a681c04f71600019a1907

you dont need bignumbers.
Just write the strings one below the other like in school and add the individual digits, overflowing to the next number.

nice without the known of us, we had the same solution without loops :-)

i have my solution! My message is, that this solution here is a Fail.

this Soltion lags with the following (only fifth, or 9,9,9,9,2,2,2,2,1)

Hmm very cool 3 liner, but little bit slow.