This is likely the best performing one.

It would not make it O(n^2). It would still be O(n) because it's looping through the vowels (a constant known length) for each char in the input, and we're still only looping through the input once. The constant in this case would be at least 5, though, so it would run in approximately 5n time.

This is surely clever, but probably shouldn't be used or marked as best practice. I'd have to performance test to be sure, but allocating an array on heap seems like it would be much slower than a branch with no array allocation. Maybe I'm wrong, in which case this would be a nice answer, but the bitwise and will always be better for both performance and readability.