Ask HN:How to understand advanced topic in programming language ?

hello everyone, it's been 3 years that I'm programming, but until now I find dificulties to understand dificult algorithms, such as recursion, quicksort, and lots of other stuff, for example the speed of algorithm, so I solve the prblem that is in projeteuler this website but in more time , as I have not done the mathematics in the university, I think I still have gaps. whats kind of advices can you give me . for example of mathematical book or topics to help me to solve dificult algorhitmes. thank you very much