I am very good at linking old stuff to a newer problem (thats why I like problem making too). If I have solved say 10 tough problems and you give me a problem thats twice as tough as them but is similar in some aspect to some of them I am most likely able to solve it. But since memory is limited as I practice, I want to keep standard problem ideas in my mind only, around 50 problems for each data structure and 300-400 problems in total. (I will obviously solve more problems but will keep on discarding the problems which I will find similar to my existing knowledge).

What is the best source for such problems is it CSES problem set, TLE Eliminators, or something else ?

Any help would be appreciated.