Monday, November 08, 2010

How to conduct a technical interview for software engineer?

(This article presents my thoughts on how to effectively conduct a technical interview for a software engineering position. It presents the "interviewer's" point of view based on more than 30 technical interviews I have conducted, and quality of candidates I have recommended. If you are an "interviewee" I suggest you look elsewhere, e.g., interview questions.)
  1. Know the position you are hiring for. If you have been part of a software engineering team or have read the book, "The mythical man-month", you would know that you need several different "types" of members in a successful team. You need a "magician", who knows or can figure out solution to every technical problem you may have. You need a couple of "plumbers" who are willing to fix any broken software piece. You need a "general" who is very motivated about what you are doing, knows how and when to delegate, and keeps everyone together. You need a few "soldiers" who can follow orders, do the job, and be happy to contribute. And so on. As an interviewer, you need to know what position you are hiring for? You need to tailer your interview as per the requirement. One interview pattern does not fit all types.
  2. Do your homework. Before the interview, thoroughly read the candidate's resume/CV. If she has extensive work experience, identify only one or two of her past projects to focus on. If you have even a slight doubt about her programming ability, prepare a written programming test. If possible, scheduler a separate or additional time slot before your face-to-face interview for the programming test. Do not use any existing online programming test material, otherwise you won't be able to distinguish between someone who knows how to program vs someone who has gone through many web sites containing interview questions. Do not give take home tests. Do not share your programming interview questions with other interviewers in your organization.
  3. Start with knowledge questions. During the interview, after initial introductions, start with a question on her past experience. Your interview should balance between knowledge and application types of questions. Do not ignore his experience or knowledge, and do not focus only on his experience. Getting started with what the candidate already knows is also a good way to make her comfortable. You can ask something from her past project, e.g., "Describe in one minute what you did in XYZ?", or ask about a past technology that he used extensively, e.g., "Did you use STL in C++? What are the common STL classes available?"
  4. Focus on real application problems. Most software engineering positions require applying your existing knowledge to a new problem. The one quality which distinguishes a good programmer from a mediocre programmer is that a good programmer can easily translate your problem in to pseudo-code. If you are interviewing for "soldiers" and not "magician" or "general", avoid discussing high-level design type of problems, but instead focus on more low level real technical problems. For example, instead of asking "How would you design a scalable web server for blah blah?" ask more specific questions. In my experience, people who can answer high level design questions can create "vaporware" but those who can translate a small real problem to pseudo-code can actually write "software". If you need software engineers, avoid wasting time on high level design questions. Also, such application problems should be independent of specific domains but just be able to test whether the candidate has the required mathematical and computer skills to translate your problem to pseudo-code. I have given some examples later.
  5. Follow thought processes and provide hints. If you believe that the candidate is getting diverted in to incorrect answer, there is no harm to give hints or counter-questions to course correct her thoughts. Do not be too adamant on your answer. Sometimes, a 75% correct answer is good enough.
  6. Provide itemized feedback. When you submit your recommendation to the HR or your manager about a candidate, specifically itemize individual qualities and performance, and emphasize specific skills and lack of it. For example, "I had a nice 45 min conversation with XYZ, and I found her to be a very good programmer but needs training on Flex. After initial introductions, I asked one algorithm and three programming questions. She did good in two programming ones and average on others. Programming ability: very good; Needs hand-holding: yes; Algorithms: average; Strength: programming; Weakness: Flex; Recommendation: weak accept." My final recommendation is one of strong-accept, weak-accept, weak-reject, or strong-reject, with implied meaning of "a very strong candidate, and must hire her", "a good enough candidate, but won't argue to hire him if others disliked her", "an average candidate, but won't argue to reject him if others strongly liked her", "a poor candidate, and must not hire her", respectively.
As an interviewer you would be wondering about examples of real questions that would distinguish a good programmer from an average one. These are some examples. As mentioned before, you should create your own question, instead of using these, otherwise you cannot distinguish a candidate who genuinely solved the problem from the one who has read this blog.
  1. Video conferencing layout: suppose you know the window dimension, WxH, and want to fit participant videos in MxM tile. Each video has fixed aspect ratio of 4:3. All video objects are of same size in the layout. Your MxM tile should be laid out in the middle-center, with potential empty spaces near window edges. The layout should maximize the size of the MxM tile, so that the empty spaces near edges are minimized. You are given an array of video objects V[] and a function layout(v:video, x, y, w, h) which lays out a single video object with size (w, h) at position (x, y) inside the window. Write pseudo-code to layout participant videos. (Hint: start with 1 video, then 2x2, then 3x3, then generalize. Additional questions: how would you modify it to NxM tile instead of MxM? What should happen if number of videos is more then 9 but less than 16 -- which boxes are empty? How would you modify it so that empty spaces including empty boxes are minimized in NxM layout?)
  2. Path optimization: suppose you have a map of a city with Manhattan-style layout. Suppose north-south streets are named, a1, a2, etc., and east-west streets are named b1, b2, etc. Some streets have traffic signals, with 5-second walk sign, 15 seconds count-down to continue walking if started, and 20 seconds don't walk sign, periodically repeating in that order. Other streets do not have traffic signal, in which case traffic must yield to pedestrians. Suppose you need to walk from corner of a5/b5 to corner of a7/b10, and only street with traffic lights on you way is a6. You walk at the same speed. Crossing a6 takes 15 seconds whereas crossing any other street takes only 5 seconds. You do not want to cross a6 if you know you can't finish before it turns to don't walk sign. You want to minimize the time taken from source to destination, hence minimize the time waiting on traffic lights. You have function named walk(), turn(left or right), stop(). Write pseudo-code for your decision process from your source to destination point. (Hint: draw out the map first, then it becomes easy to visualize and solve. Additional question: can you generalize between any two points as long as you know the complete map and which streets have signals?)
If you have more ideas, feel free to comment.

1 comment:

moozai said...

Hi

Tks very much for post:

I like it and hope that you continue posting.

Let me show other source that may be good for community.

Source: Engineering interview questions

Best rgs
David