http://codeforces.com
https://polygon.codeforces.com/

https://github.com/MikeMirzayanov/testlib
https://github.com/MikeMirzayanov/olymp.sty

https://codeforces.com/blog/entry/512?locale=ru

archive
http://codeforces.com/problemset

blog collection
http://codeforces.com/blog/entry/1395

how to learn problems solving
http://codeforces.com/blog/entry/4080

misc
http://codeforces.com/problemset/problem/1/A

geom progr
https://codeforces.com/problemset/problem/1348/A