Wow! This one is what I was looking for. Simple and elegant. But my brains didn't even turn into this direction.
Any chance you can go over your thinking process into figuring this out? Is it something math knowing people know?

This solution is very idomatic and easily read. So it deserves it's spot at the top.

Your solution has a hidden assumed limit and a bug on the ammount of any specific symbol appearing in the string.
If s1 contains MAX_INT or more appearances of a single character your solution starts returning the wrong output.

Yes, I saw what you did there. You have avoided explicitly generating local copies for sorting.
Which I did in my solution so I shut up about it. In any case I like your solution for how readable it is.
I'm still hurting for failing to find std::includes() in the docs :-) I think maybe to big for my head anyways.

Modified function signature removing the constness of input parameters.
I will call this cheating just because of me being angry and envious. I never new std::includes() exists.

This was my first thought, but I have decided against it.
I'm here to practice writing easy code and use the new stuff that I have never touched in the language.