read pseudocode definition of the algorithm and implement in your language of choice, alternatively you could port from one language to another language

In which case I'm curious why it's an unreasonable interview question.

The argument is always "use the stdlib one", but if there isn't one, and you need to implement it, why isn't it fair game?

it's fair game, but to have the best shot of implementing the algorithm the correct way would be to look up the algorithm definition in a reputable algorithms book and implement directly from that, doing so also provides the developer with additional knowledge and context about the algorithm's use and performance characteristics, my 2 cents

Replace "algo book" with "person asking you the question", and I see relatively little difference.

One difference is that this penalizes people with social anxiety.

Don't all in person interviews do this?

Some more than others. It can be mitigated, and probably should be for certain roles.

I guess it depends on what you're interviewing for. If you want a junior to implement specs he's given your point might apply. It's not an appropriate question for someone with basically any real experience, though. Especially not someone who you expect to do anything more complex, like engineering.

So now I ask this:

Given that google probably does more software interviews than you do, why do you think they ask such questions, even to experienced candidates?