I found this situation and think it is not well.
I think this is not right. Is it right? If it is not, how can I notify it? How can we solve it?
This is the link that generated my question.
If the answer differs substantially from the originally accepted answer, or combines several different answers into a single one, then I don't see a problem. The OP apparently found his answer more useful.
If the new answer is essentially the same as the original answer, I would leave a comment stating that you feel it is bad form to hijack someone else's answer, and downvote his new answer.
If it's basically a copy/paste of another answer without attribution (crediting the original author), you can flag for removal, citing plagiarism.
It's not cool, but in my experience it happens rarely enough. It's happened to me maybe 1 or 2 times - with about 7,000 answers authored.
If it happened to you, leave a comment asking the OP why they did it.
If nothing comes out of it, let it go.
Well for 1 I don't think that the OP get any reputation from accepting his own answer as opposed to someone ease's answer(at I'd hope not, it's a bad incentive).
It's a mean thing to do, it's essentially telling you that your answer's not good enough to deserve to be marked, but there's really not too terribly much you can do about it. he's free to determine the answer to his own question.
And after a cursory look, his answer looks a little bit different from yours(it's longer for 1)., and if he believes it to be substantially better, than he's within his rights to mark his own as the answer. A lot of the point of the marked answer is for people who come here googling the problem, not for the glory of the answerer.
I have answered my own questions before, but ONLY if nobody else has provided a correct answer at the point where I have gone ahead and found my answer.
"Well for 1 I don't think that the OP get any reputation from accepting his own answer as opposed to someone ease's answer" It's the loss of rep for the OP of this question that bothers him, and the other asker could still potentially get rep from upvotes. "It's a mean thing to do, it's essentially telling you that your answer's not good enough to deserve to be marked" It's not mean if he really doesn't think it's as good of an answer. If he came up with something better marking his own answer is certainly appropriate and not mean at all.