These challenges are meant to give you some examples of something you might see on an interviewing challenge. But more broadly, to help you stay sharp and help clarify some of the content on the Cheat Sheets main README. The challenges are divided into different languages (if you don't see yours below make a pr).
The way they work is simple. Open up whatever language you're interested in and follow the README. As a note, the tests are designed to be followed in the order they run.
Each language will have the following directory set up in common: (any other files you see are to simply make it work)
<language>/
|- README.md This explains how to get it working on your machine and how to run the tests
\- challenges/ This directory has the challenges broken into different files, waiting for you to make them work
\- solutions/ This directory has the (suggested) solutions to the challenges in the same file structure as challenges/
The plan is to have the same challenges in multiple languages to help you expand or refresh your memory on how to use one. However, I can't do them all myself, so if you see your language is missing feel free to open a PR and contribute them. My only rules are use sensible variable names. Naming all your variables one letter isn't helping anyone, and it inhibits learning. And follow the set up for how the existing ones work.
Many of these exercises are taken from Khan Academy's Fantastic Course on Algorithms. I highly recommend it if you're feeling lost or just want to learn more in general.