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...."

This is very clean

Thanks!

I like your code!

index - 1 is what'd I'd go for in production. I used it here, because I wanted to move on to the next problem and didn't want to spend time adding -1 to every value.

That's the reason I didn't vote his as best practice.

Why "count" have 7 indices if you only use 6 of them? You can count the numbers by their value minus 1