Wow. this is one of the reasons I hated school. No programmatic reason what given for why a string solution couldnt be used, only an arbitrary reason. Here students may have knowledge from self teaching or whatever, but they are unallowed to use that knowledge because "reasons".
To any teacher that thinks its a good idea to punish students for thinking outside the box: shame on you. All youre going to end up doing is crushing enthusiasm and/or creating drones. Please dont.
The entire problem is arbitrary. It's not a real-world problem looking for a solution. It's a programming exercise.
These crop up all the time in those online programming challenge things.
They are usually maths problems (e.g. Find the combined area of two partially intersecting squares based on their coordinates of a,b,c,d and w,x,y,z) where the solution is done via programming but most of the work is, in my opinion, figuring out the maths which is testing for the wrong thing.
Notice that all the straightforward solutions the author rejects, they reject because it fails on 0. Then, with the oh-so-clever log10 solution... surprise! They special-case 0. I'm not sure what the lesson here is supposed to be.