For coders who have learned computer science for years, there should be many hard problems you encountered in Codeforces. However, only some of them are memorable. Each of those problems became a vertebrae in the whole backbone which you lean on to solve other problems e.g. When I encountered this problem on spoj, I learned the idea about how to apply persistent segment tree and binary search on find k-smallest number => it helps me a lot, for example, in solving the problem E in Regional acm icpc 2016 in Bangkok.
Because there are tons of problems on codeforces and core problems are not that plenty, I want to create a blog that sum up as many core problems as possible to help learners, from juniors to seniors, to shorten the time spending on finding good problems.
So, please comment some problems that, in your perspective, are core problems.
Thank you!!!
Waiting for years for someone to post this :D
shitposting
I really liked this problem for one reason first of all when I solved it I felt really smart.
second it was of the first E problems that I have solved and it broke that fear of mine about problems of E difficulty.
337E - Divisor Tree
I hope this problem will be nice for the rest of you guys.
:)
This problem
It was the first problem where I had to use two pointers to solve it and now I can almost always solve them in contests.
Check this problem on Codechef on euler totient function. It taught me concepts like summation of i^k (asked in educational rounds), lagrange interpolation etc.
You can find my implementation here. Was quite happy to get the best solution on this problem :)