Can you provide the expected output of each question and compare the output one-by-one, calculate the timing for them?
And generate a evaluation report in GA, so people coming around can check the report, instead of running by themselves, and understand the state of the repos.
Made yourself a like this or this maybe.

Understanding how branches work is essential to work with git.
Be realized about you are working on which branch, and what are the roles of each of them.

Usually there is a default branch you want to show to other users, which is main by default "now". Legacy docs might refer to master due to some historical reason.
Am fine with either. Just be consistent, and let gh know by going to project setting somewhere.
gh is using main, while I see you r push something to master.

And you have a mid branch. Not sure what s the purpose of it. You can explain the role of each branch (that is your branching model) some where other users can see.
For branching models that are more commonly used, you might want to consult the following links
https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow
https://nvie.com/posts/a-successful-git-branching-model/
https://docs.github.com/en/get-started/quickstart/github-flow

Again, either model should work, just be consistent and declare somewhere so other users can understand.

Think you have a plan to post two solutions every week. But it is yet empty here. What are you waiting for 😃

It is great you are making progress and getting familiar with it. Let's be looking at something more fun.

Please consider making the solutions compilable and runnable for validation from the command line.
This is an essential step in making the automatic pipelines. You don't usually have UI when working with (economically efficient 😄 ) automatic things.

What is more important, it could be a lot more fun here.

For example, there could be a makefile managing the compiler commands, tackling problem selection, and even automatic testing.
Possible semantic:

# To compile the solution for question 812
make 812
# To test the solution for question 812
make 812-test

As you are using Visual Studio, you can figure out its compilers from manuals or google for it.
You might need a sophisticated configure file or cmake to make it robust and feasible for different environment settings. Try to figure it out if you need it.

It might be a significant amount of work. So please consider doing it in a git branch, continue the current exercise with your current branch, and merge (with a nice PR) when the change is ready.

You are making progress 🔥

I see your concern in testing the windows-based code on linux. But most of the source code won't depend on any OS thing.
If necessary, keep an OS dependent set of headers and make sure the source code for each solution is compatible with either windows or linux.
Then you may use a linux-compatible compiler in the gh action while still play with windows locally.