Hello guys! I have been training on codeforces for a while, and noticed that up until 1600 (currently I try to solve problems around that range of difficulty) there's not very much use of heavy algorithms and data structures (the heaviest I've used in codeforces so far is DSU), and the problems revolve mostly around thinking, observations and so forth.

For the moment I plan to continue training on codeforces, but further in the future, closer to ICPC regionals, I want to train with problems where more techniques and algorithms are used, so I would like to know which problem archives / websites / types of contests do you all recommend for this type of training. Even better if it's older ICPC problems

I saw BenQ's template GitHub and saw his ModInt template (https://github.com/bqi343/cp-notebook/blob/master/Implementations/content/number-theory%20(11.1)/Modular%20Arithmetic/ModInt.h) and I was wondering if someone has made a similar template to support huge numbers using strings (which can support every operation that ints or long longs support but the underlying structure is a string).