Do you have any test case to prove it is a problem?

I know the difference: your solution will make both branches execute if func1() returns something not truth-ish!

I think you might run into a problem if two of the numbers are the same.

language?

I'm getting the same issue, it passes sometimes and other times this or a similar incorrect test shows up.

Sorry. That was my mistake.

Yes, I would very much like to know the difference as well... Have you tried stackoverflow @vlakas

the most human readable solutions always is better

I think that this is clever but not best practices at all.
E.g. you have chars = ['A', 'B', 'D']
Instead of 2-3 loop cycles you need to make 26 cycles for lowercase letter and then 2-3 cycles for uppercase letters.

Thus we do not have to maintain counters of chars at all. I think your comment is very useful one.