Status: IN
Hash-based data structures (Counter for frequencies, set for membership) serve as the universal O(n) preprocessing layer, with nearly every lookup-heavy or frequency-dependent problem beginning with one of these two constructions before a linear scan.
Counter and set are complementary hash tools — Counter for "how many" and set for "is present" — and together cover the preprocessing needs of the majority of easy/medium problems
Depends on (SL): counter-universal-frequency-primitive, set-for-o1-membership-universal