I am not sure, if isalnum() is faster, but the code would be much shorter and more readable. But where is the fun. ;)

I didn't even think to use continue statements, so useful.

Is there a time complexity difference between using the if statement here vs. just using the statements in ctype.h? (just a thought)

C++ seems have the same problem.

Fixed

@Toideng it did receive two size arguments cuz the author's solution was adapted to solve m x n
And usually that is the case with matrixes.

Changed paramerer names to be clearer. Unfortunately, the outsz setting has to remain because removing it will invalidate all the solutions.

In C, type of arguments should probably be int64_t or uint64_t since their values are said to reach as high as ~2E+14, but int can only contain ~2E+9.