I took these two OCW courses: https://ocw.mit.edu/courses/electrical-engineering-and-compu...
https://ocw.mit.edu/courses/electrical-engineering-and-compu...
And worked my way through these two books: https://www.amazon.com/Algorithm-Design-Manual-Steven-Skiena...
https://www.amazon.com/Algorithms-Robert-Sedgewick-ebook/dp/...
I wrote out my solutions on a whiteboard or pen and paper before typing them into a computer.
Outside of A & DS, I spent some time on operating systems and networking in general. A little bit of probability, but no hardcore discrete math.
There are swaths of areas which I now cover at a slower pace as time allows; compilers, AI, etc.
that seems like an incredibly short amount of time given the content. But if true, congrats! and please let me know your secret!