:-) note the fmt::join up there is IMHO canonical in the 21rst century..
Know your STL and get rid of those for loops.

Finally, the universal C++ solution after 16 months of the original kumite. (15 months since my C++ translation.)

• default solution should inslude only <string>
• add PUZZLE tag

Yop, waiting for C++20 full compatible compiler in Codewars (C++17 supported), rangev3 is supported as external library .. See github repository "using rangeV3 in codewars".
A bit verbose in the header files, but more convenient...
Was tired to type "c.begin(),c.end()"; and the ::views can be really nice on some algos.

Ranges library has first been introduced in c++20.

minor correction: use const reference, to make sure you are not changing anything on the data structure.

Whenever passing vectors, or any STL data structure to a function, always use references.

Use Pass by reference instead of Pass by Value.

This would reduce the time in copying your vector.