Just a question regarding answering with a library.
Function that adds two integer strings question is basically a home made algorithm for parsing and adding to arbitrarily large strings with integers together.
Should you answer as you would/should do in real life? (Use existing libraries) or should you always answer as if the code in the question is the correct way to do it, and improve on it? May be rephrased as:
Should you answer and review the intent of the question, or the actual code?
My answer is: https://codereview.stackexchange.com/a/99088/15532 and even though it might be obvious for some I argue that some people might not be aware of BigInteger and the set of answers is more complete with this answer as well.