Did he explain why your answer was incorrect ?

No. The answer I came up was bit complex as compared to his answer so he just gave me a couple of hints in the direction of his answer and then I figured out his solution and he moved on.

If the interviewer is going to reject you, emailing him now won't stop that happening. If he isn't going to reject you, emailing him now might stop that happening.

@LaconicDroid So you mean if I mail, I would definitely be rejected. That seems unfair.

@Kaushal28 - no, I mean if you are already accepted there is nothing to gain by emailing him, but a slight risk of changing his mind. If you are already rejected, nothing you do at this point will change his mind.

Does this answer your question? How to deal with interviewers who are mistaken during job interviews?

To give inspiration for more thoughts. The way this interview went could be a hint to how this company is organized and run and how decisions are made. Would this shine another light on how bad it would be for you if they rejected you for this email?

@Kaushal28: interviewing isn't an like college examine. Yes, you have to show enough technical capabilities to do the job, but behavior, cultural fit, communication skills are often as or even more important then your technical stuff. Getting back to the interviewer would indeed be a mistake.

Not an answer because it's just my own personal feeling but...as someone who occasionally gives technical interviews; I would absolutely welcome a follow-up email. I don't want to be wrong, and I especially don't want to be wrong in a way that impacts someone else's career. Plus, a large part of what I (and others on the team) do is fix mistakes we have made. Being able to convince me that I was wrong, especially if you can do it diplomatically, is a huge win in my book. It wouldn't just increase my opinion of their technical abilities but it'd be a chance to show lots of other good traits.

In general, a good interviewer will want to see that youhave a logical approach to a problem, that you think about it, ask questions about assumptions, see that you have a thought process in mind. It doesn't have to be the exact way they do it, unless you have a bad interviewer, in which case you can't do anything about that.

I'd have a positive reaction to a follow-up that said "Thanks again for taking time for the interview; that one question about X bugged me the rest of the night but I think I figured out that Y does work, though your solution would be more performant. Just thought I'd share, thanks again!" On the other hand, "Hey I now have PROOF that you were UNFAIR to me during my interview; my answer DOES produce the correct result!" not so much. The point? If you do choose to follow up, pay attention to the phrasing. I wanna hire someone interested & positive, I don't want a whiner who has to be right.

Do you want to work for pigheaded people who refuse to accept that most problems have multiple solutions?

I'd rather put that on my blog or somewhere than feeding back to them.

Sympathies and a related memory: madmath.com/2015/12/that-time-i-didnt-get-job.html

I have done hundreds of technical interviews over the years and have had a fair few "MY SOLUTION DOES WORK" emails. I can't think of a single occasion where the email has changed my mind on the decision, and in almost every case the issue was not in fact that I mistakenly didn't believe that the candidate's solution would work, but because the candidate had not properly understood the question (perhaps they missed a constraint, or made a false assumption) and despite my attempts to explain the angle they were missing they could not see what I was getting at.

As an interviewer, I would appreciate knowing that I'm mistaken somewhere so that I don't unfairly disqualify knowledgeable candidates. But most interviewers are not like this. The industry has a lot of people who are completely ignorant and a lot of people who will go to great lengths to gatekeep. I'm not sure it's worth following up -- but if you do, be polite and cool about it.

There are also a lot of interviewers who ask questions from a list and don't really know how to deal with it when the interviewee doesn't give the official answer. Unfortunately you can't always get subject matter experts with great people skills to do interviews. Even if it's not an HR drone, it might just be whoever's available that day.