I like the code review approach and tried it a few times when I was needed to do interviews.

The great thing about code reviews is that there are LOTS of ways people can improve code. You can start with the basics like can you make this code run at all (i.e. compile) and can you make it create the right output. And there's also more advanced improvements like how to make the code more performant, more maintainable, and less error-prone.

Also, the candidates can talk about their reasoning about why or why not they'd change the code they're reviewing.

For example, you'd probably view the candidates differently based on their responses to seeing a code sample with a global variable.

Poor: "Everything looks fine here"

Good: "Eliminate that global variable. We can do that by refactoring this function to..."

Better: "I see that there's a global variable here. Some say they're an anti-pattern, and that is true in most but not all cases. This one here may be ok if ..., but if not you'll need to..."