the description says the input is always valid

What if there is unicode char as the first element?
What happens if the input is an empty string which is then unwraped?

Almost did the same thing...but I changed the trait bounds on array_diff to also require that T implement the Copy trait.
Then I used iter and copied instead of just using into_iter like I should have...because I initially misspelled into_iter while trying this exact thing :P

best practice 🙏

I don't think that it's one of the best solutions: here filter is called two times, so it's likely that you iterate two time on input, which is unnecessay. This can be solve with a vanilla for loop & one if.

Rust is a pass by value language so really this is only mutating the function local value.
As long as the input variable is not a reference there's no risk of mutating the passed in value from the perspective of the caller.

Do you know you can fork the solution and see what happens if you remove this part? Just click on fork below the code. Rust compiler explains things very well :)

Yes, but array sizes are quite small in test cases, so creating additional structures can be less productive in this specific case.

This code has O(n*m) runtime as for each element we have to re-scan in b from scratch.

Great solution 👏👏

nvm it was just a typo.

This compiles and runs perfectly. Could you copy your code here?