Go for Grokking Algorithms book by Aditya Y Bhargava, basic and easy to read.

I think algorithms and data structures course by Pavel Mavrin is the best course about algorithm. Laboratory work is also attached to this course, but they seem to be only in Russian. I watched this course in full, but only in Russian. I highly recommend it.

There is also many useful practice site such as cses or USACO

Also there are good theory site such as itmo wiki(only in Russian) or emax(only in Russian, but I'm sure there is a similar analogue in English)

Hi Rimmam. I noticed that participating contests and solving all the problems after the contest help us very much. I think watching total code or total solution would not help us. Even though you solve 1 problem for a week or nothing, it is good only if you really tried to solve it out. Solving A, B, C on Div2 does not require advanced algorithms and data structure, it requires logic, and finding core clue. If you solve one problem, then you should try to teach your solution to other, people learn while teaching. You can find a friend who is enthusiastic and energetic on this codeforces.

Maybe I am wrong.

Thanks for hearing me out.