Well, every time I read an article, it is difficult for me to sum up and get the point.
So I just read, but there is nothing left in my brain.
I think it's not an effective way to learn new things.
To improve the situation, I watched some videos and tutors.
These are the sources I find:
1. https://www.youtube.com/watch?v=VwEl-MiZH0E
