I love when all planets after 7th are neptunes!

it's passed by reference because pointers are cheaper and faster than making a copy; and it's a const reference because the function does not need to, and should not, modify its input.

my question is why the h is passed by refrence, and why it is also constant?

You can do the return (and using the i for the index) inside of the if, I think is better because the return itself ends the function.

Oh, sorry, forgot that part

Given a lowercase string

HashSet

Will fail when there are uppercase letters, but clever solution

Would be a lot better to use StringBuilder, that thing is no joke 100 times faster