Note: this solution contains undefined behavior!
The signedness of char is impl-defined, while overflow of a signed integer is undefined.

The lexer and parser parts are very elegant!

Why? I see on cppreference that it has been in standard since C++11. Since the solution has already tested if the result is empty, I think there won't be any undefined behavior here.

No I can't.
I tried to use operator() and operator int in a class to simulate a function but it just does not compile. Wonder if someone compiles.

This solution has a taste of functional programming

My solution is bad because there is unnecessary string creation. It's okay to just modify the strings in arr themselves since std::string is mutable, unlike strings in C# and many other languages.

I met with the same problem in Rust. I thought there would be only SPACES before and after the string so I used .trim() to remove them, which worked quite well in C#, but I still couldn't pass the tests. I saw the solution ranking No.1 used filter map to eliminate the invalid input, which I guess shouldn't be necessary, so I wonder what the test cases are.

Very concise solution!

this solution works but does not do what it is expected to do, so I re-submitted it.

This solution is rather structurized and I love it!

Love this!