I am working on Segment Trees, and more recently with this implementation by Al.Cash

He brings up this problem but even with the segment tree already built (I assume it stores the number of 'c's in a given range) I don't know how to answer the queries in O(logn).

I would appreciate some help with that problem, and recommendations for other good segment tree problems. Preferably on Codeforces (there is no segTree tag; data structures is the closest but it's no guarantee).

Thanks and have a good day!