Diff two maps in Rust
Rust interview exercise on Practical, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write diff_keys(before, after) that returns a Diff of key names: added (only in after), removed (only in before) and changed (in both with a different value). BTreeMap iteration keeps each list alphabetical. The given map(..) helper builds test maps.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.