I have seen few question which were updated by original poster and added solution to his problem. Is it fine to copy his solution to answer? Or should I just flag it/delete it?
I always leave a comment on these asking them to post their solution as an answer. I can't remember an active user ever failing to comply, since they stand to gain more reputation for the answer. If you've already left a comment with no reply, and it's clear the person is just not coming back, I'd say go ahead and leave an answer. I'd put the original comment in quotes, just so it doesn't look like you're trying to steal credit, but the more you can add beyond their original comment answer the better.