https://codeforces.com/blog/entry/20548 https://codeforces.com/blog/entry/20548 is old but pretty good. for e.g. #2 is specific cases: "you get a problem for a

https://codeforces.com/blog/entry/20548 https://codeforces.com/blog/entry/20548 is old but pretty good. for e.g. #2 is specific cases: "you get a problem for a tree. Consider its variant where the tree degenerates into a path".