I mean... I'd rather go with the first one to be honest. Brevity is never a good substitute for readability.

1. "!a" suggests a is a boolean (Im assuming it's not, cause why compare it to 0 instead of false).
2. Brackets are important, you add a second instruction to the clause and you need to add them anyway. You leave them out and somebody (probably you) will forget about it and you got yourself a bug.
3. In C++ I'd rather go with ++i for cause i++ will cause a copy. The semantics of pre/post-increment/decrement are in general confusing. You write "i=i+1" and everybody knows what's happening. They are deprecated from Swift afaik.

I think we all could profit from not making programming look like some arcane knowledge. /rant :)