What to do with unanswered questions that have an answer in their comments?

Question

This site has hundreds of unanswered questions, and that doesn't look good.

It isn't hard to find unanswered questions that in fact do have an answer though, it's just that the answer is posted as a comment, not as an answer.
For instance: tic tac toe - Quantum tic tac toe strategy.

Is there anything that could or should be done with such questions?

Answer 1

^{This answer might be subjective}

When the answer is posted in the comments, it can mean the question is trivial, but for the post in question, the "answer" is actually a post on another Stack Exchange site.

This can be because of two things:

• Non-original puzzles with answers already posted on their source
• Lack of research to confirm the puzzle isn't recreating something already made

For the post in question, I believe OP didn't know about the other Stack Exchange post and wanted to either make people more interested in Quantum Tic-Tac-Toe, or simply offer people a nice strategy problem that many people enjoy.

But to me, it's not only OP's mistake here. The author of the comment in question just Googled the question, found the answer, and then posted it as a comment. Both users are at fault: OP for not double-checking if their question were original, and the comment author for not giving OP's puzzle a chance on this site.

If such a question, or answer for that matter, arises again, I think it's fair game to either notify OP in a friendly way that their puzzle has been answered on a sister Stack Exchange site before, or answer it with the newfound knowledge the comment author googled. I don't think one should google puzzle names or such just to find the answer faster. Because by "solving" puzzles this way, you cheat not only the puzzle, but yourself.

TL;DR

Ignore the comment, do your research and solve the puzzle like you would with any other.
(But notifying OP their post exists already if they're not aware of it is fine)