Generic group_by in Rust
Rust interview exercise on Traits & Generics, difficulty 3 of 3. Solve it in the editor beside a worked example; the tests run the moment you do.
Write group_by(items, key) that groups clones of the items by whatever key(&item) returns, keeping input order inside each group. K needs Ord because the result is a BTreeMap; T needs Clone because the input is only borrowed.
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