Getting runtime error for no obvious reason

I am submitting code for biggest number problem and I am getting a runtime error. I don`t see why I should get that. Please see my code on the following link:

if the numbers are same and equal in length, you get a seg fault. because then you access b.at(minlen).

I hope I’ve cleared your doubt. I ask you to please rate your experience here
Your feedback is very important. It helps us improve our platform and hence provide you
the learning experience you deserve.

On the off chance, you still have some questions or not find the answers satisfactory, you may reopen
the doubt.