As I understand this solution, it will allocate memory for characters after the anchor '#', so we can use less memory here.

yes, you're right.

It's a better practice to make 'i' the same unsigned type as 'a'. As an int, 'i' could underflow for large values of 'a'.

Probably because ChatGPT was trained on code like this. This solution is from March 2019 so it predated publicly available ChatGPT by a few years.

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look like ChatGPT's answer

Thanks! I did not know this.

This is a nice solution, there's a particular pattern for the rules. I solved the problem, but the way I solved it is specific to this problem.

I have smillar solution ;)
One tip that comes to my mind is that you don't need to put 7's zeros in count initalization, it is enough to just put one zero and the reset is intailzed with zero too:

int count[7] = {0};

see https://en.cppreference.com/w/c/language/array_initialization
"When an array is initialized with a brace-enclosed...."

Хмм, а мы парились с быстрым поиском делителей, сенсей (0_0) :D