• Sample also does not show the expected result and gives away the function name

PS: Happy Chinese New Year too, but you can always express it in words instead of a kata OwO XDDD

• string header is not included
• Initial solution is using string instead of std::string
• The parameter type should be const std::string &
• The reference solution shouldn't be preloaded
• No random tests
• Concatenating strings is neither novel nor interesting kata idea