I created a video editorial for problem F. Sum of Fractions from Educational Codeforces Round 188 (Rated for Div. 2)

The video starts with a bruteforce approach $$$O(N^4)$$$, and then subsequently optimizes it to $$$O(N^3 \cdot Log(N))$$$, then to $$$O(N^3)$$$, followed by 2 versions of $$$O(N^2 \cdot Log(N))$$$, and finally the intended $$$N \cdot Log(N))$$$ solution (ignoring the log factor from modular divison).

Here is the video link https://youtu.be/8ixZezwq_20?si=i3ChK6zAEZKf6nZG

I also curated some practice problems and created easy version of problem F so that you can submit to verify the correctness of your slower solutions. You can find the contest here https://codeforces.me/group/7Dn3ObOpau/contest/537569 (Note, some problems are already added, working on the remaining ones, check back after a couple of hours).

The problems are untested, if you see any issues, do let me know.