Well, Codechef and Organisers get a LOT of criticism. So I thought why not share some positive thoughts I feel right now.
Thank you Organizers. Thank you Admins.

Chef and Dragons: First time I used Seg Tree with great efficacy in a proper contest. The moment I saw this  I realized that this is a STACK+SegmentTree+Lazy problem. I was halfhearted about attempting the contest but this one drew me in. Thank you for boosting my Segment+Lazy confidence.

Weird Mul: I saw the rolling hash polynomial nature of the problem. I understood modular divisions rather well. I couldn’t solve this problem, but I could see how a given weird multiplication is like a substring.

Expected Repetitions: I couldn’t solve this problem fully, but now I am aware that there exists a thing called Ukkonen suffix tree array. This is what I am going after next.

Kirchoff’s Theorem and how spanning trees can be calculated from a matrix. Didn’t even attempt the problem though.
I now understand string hashing a little better.