An answer that may not exist in Rust
Rust interview exercise on Option & Result, difficulty 1 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 average(nums) that returns the mean of a slice of f64, or None when it is empty. Dividing by zero would give NaN; None says what actually happened.
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.