Why this code is running even there is an error?

Question

This code contains the usage of strcat and strrev functions, but doesn't contain String header file. But still it runs and gives correct output. How is this possible?
https://code.sololearn.com/cRH33mBg7z4V/?ref=app

Answer 1

Maybe the libraries are included by default server-side, so users don't have to do it themselves...?