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!!!