Shor the Duck's SG Collection, a curated problemset split into difficulties of the best problems on Codebreaker set by the SG IO Community!
Criteria for 3 star:
Requires some simple application of some basic technique/data-structure, or thinking that may be non-obvious to beginners.
(Binary search, (not including BSTA), Prefix Sums, Linear DS, Two Pointer)
Should remain suitable for Codeforces pupils that have learnt the basic content.