Hello guys, if you analyze a lot of past codeforces problems in div2 you can notice knowledge of standard algorithms/ds is useful but not necessity. That segment tree solution can be done with set, or with prefix sum style sweeping.

Actually the most important "algorithm" in these problems is using your brain as someone said here.

That is why I want to ask the community for some great ad-hoc problems that have made you think/taught you important concepts or observations. (For example, you can add X to a set of numbers in O(1) by keeping global variable) I think these skills of making observations/simplifying the problem/looking at the problem from different angles is something me and many other coders are lacking, and it would especially help someone like me who is good at standard stuff but quite lacking in problem-solving skill.