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Personally, I would feel trying to find the solution by parsing other people's code would be way more complicated than just coming up with your own solution.

I would forget about C, though, and try to do it in a language you already know to begin with.

To be honest I also find it very difficult to relate, as especially the Cesar Chiffre seems like such an easy problem. I only have to read it and my brain already maps out the solution. (I guess you could call it a No-Brainer, except that it seems to be a "Brainer" :-)

I honestly can not imagine how you usually code.

However, I want to say I remember my first year of studying mathematics. Many of the exercises also seemed very hard back then, but looking back on them the following year, they actually seemed quite easy (well, most of them). So practice and experience really can work. No guarantees, though.

Personally I like "Project Euler" for programming problems: https://projecteuler.net/ I actually had questions pop up in interviews that I had previously solved on project Euler.