cracking machine learning engineer interview

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OFFICE HOURS

How I cracked my MLE interview
at Facebook

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My journey to becoming an MLE
Rahul Agarwal Oct 27, 2020 · 14 min read

It was August last year and I was in the process of giving interviews. By that
point in time, I was already interviewing for Google India and Amazon
India for Machine Learning and Data Science roles respectively. And then
my senior advised me to apply for a role in Facebook London.
And so I did. Contacted a recruiter on LinkedIn, who introduced me to
another one and my process started after a few days for the role of Machine
Learning Engineer.
Now Facebook has a pretty different process when it comes to hiring
Machine learning engineers. They do coding rounds, system design, and
machine learning design interviews to select future employees. Now as far
as my experience as a data scientist was concerned I was pretty okay with
the Machine learning design interviews but the other interviews still scared
me. I had recently failed a Google interview for Machine Learning Software
Engineer in the first round itself just because I was not prepared for Data
Structure questions.
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Later as I studied for the FB coding interview, I realized that I took it a little
light and that I was not prepared for the coding interviews at all.

In this post, I would outline my approach to all these different interviews
and how the whole process went round by round for someone who is
interested in MLE roles at big organizations like FB.
So, once I was connected to the recruiter, the next step was a telephonic
interview.

1. Telephonic Interview:
This was a very basic Data Structure interview and sort of a basic sanity
check. I guess FB just wants to give you some more time to prepare for the
coming rounds and also see whether it would be worth to call you for the
onsite rounds. For me, this interview lasted 45 minutes on a Video Call. The
interviewer started by telling me about his profile at Facebook, and in turn
asking about my profile for the first 10 minutes or so.

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Then, I was given 2 very basic array and dictionary-based problems to solve.
The interviewer shared a coderpad link on which I had to solve these
problems in any language of my choice (Not Pseudocode) bug-free without
any code format options. I was asked the Time-based constraints and the
Space-based constraints also for these questions as well. The interview
progressed with me coming up with a bad running time like O(n³) and the
interviewer asking if I can do better and giving hints when needed.
As I am not allowed to share the exact questions, I would just share some
comparable in difficulty but not the same public Leetcode questions for you
so that you can understand the difficulty level and practice accordingly.
a) Monotonic Array: An array is monotonic if it is either monotone

increasing or monotone decreasing. Return
array

A

true

if and only if the given

is monotonic.

b) Valid Palindrome: Given a string, determine if it is a palindrome,
considering only alphanumeric characters and ignoring cases.

My Plan of Action For this Interview?
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This was only the second-ever Data Structure interview I was giving and I
wanted to prepare a little after my dismal performance in the first one. So, I
started with just understanding the basics of Data Structures using the
Cracking the Coding Interview book by Gayle Laakmann McDowell. The
book contains a lot of preparation tips as well and you would be prudent to
go and read them. The best thing I like about this book is that it's very
concise unlike Thomas Cormen Introduction to Algorithms and just gives
the right amount of background for coding interviews. Every Data structure
is explained very concisely in 2–3 pages, some questions are solved around
that particular topic and then a few practice questions are given. This book

also restricts itself in a way that it only has the most often asked Data
Structures. For Example, AVL Trees and Red-Black Trees are kept in the
advanced section and not in the trees and graphs chapter as they are often
not asked very often in a time-bound interview setting.
I started my preparation by creating a list of topics I would need to prepare.
You could prepare even more topics but these are the bare minimum for
these interviews.
Data Structures: Array, Sets, Stack/Queue, Hashmap/Dictionary,
Tree/Binary Tree, Heap, Graphs.
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Algorithms: Divide-and-Conquer, DP/memoization, Recursion, Binary
Search, BFS/DFS, Tree traversals.
Then I proceeded to read about them using the Cracking the coding
interview book and solved a lot of easy-level problems and a few mediumlevel problems on Leetcode for them. There are other platforms too to
practice online but I liked Leetcode due to its good design with no ads and
the class-based programming structure to the solutions. It also provided a
good way to search for questions on various topics along with difficulty. I
also gave many Mock Interviews on Leetcode just to practice. I did this for
around a week or two, spending around 3–4 hours every day.
I also started auditing the Algorithm Specialization on Coursera by
UCSanDiego around this time period, which sort of provided me with an
idea about the sort of content taught at undergraduate universities to deal
with the coding interviews.
I also wrote some blog posts around what I learned and tried to explain it
simply. You are free to check them out at my blog.
A simple introduction to Linked Lists for Data Scientists

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3 Programming concepts for Data Scientists — Recursion/Memoization,
DP and Binary Search
Dynamic Programming for Data Scientists
Handling Trees in Data Science Algorithmic Interview
TLDR; Just jot down the topics you have to prepare for and practice a lot of
easy questions on each topic. And maybe a few medium ones too.

Once, I was done with the telephonic interviews, the recruiter returned in a
short period of 1 day and set up a call to explain the process for the onsite
interview. The onsite round was to happen in London and I was genuinely
excited about the prospect to travel to London. An all-paid trip. There
were to happen around 5 more rounds which I am going to talk about next.
I took an interview date in 2 months so that I get some time to prepare.
Anyways the VISA and the whole process took up taking a little bit more
than that.

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Once in London, I reached the Facebook Office from the hotel they
provided at around 9 on D Day. A full one hour before the scheduled time
because I was anxious and I normally try to reach interviews on/before
time(More so when I am giving them). I knew beforehand the whole

itinerary of the day as it was shared with me by my recruiter. I also knew
which interview would happen at what time and who would take it. It was
in fact the most organized interview experience I have rather had.

2. Onsite Coding Round 1

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Image by StartupStockPhotos from Pixabay

I have been in Data Science for so long that I read DS as Data Science and
not Data Structures. And this interview was essentially a pain point for me.
This is something that I learned in a period of 2 months rather than having
my whole experience around it. And here I was to be evaluated on this
rather than all my experience and Data Science background. But as told to
me by the recruiter they have pretty much-fixed processes and I had to go
through these rounds for the MLE position. So I played along.
As for the interview, It started on time and as before the interviewer
introduced himself before going into my profile for a very small time and
then straight jumping into the interview questions. This time the questions
were a little more difficult and a lot of time was given to formulating the
approach and around Time and Space complexities of the solution. I was
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asked a medium level string problem which I was able to solve quickly, and
a medium level Binary Search Problem which took most of my time but
finally I was able to solve that. Some comparable Problems(Not the same
ones) with their brief from Leetcode:
a) Complex Number Multiplication: Given two strings representing two
complex numbers. You need to return a string representing their
multiplication
b) Kth Smallest in sorted Matrix: Given a n x n matrix where each of the
rows and columns is sorted in ascending order, find the kth smallest
element in the matrix.
The interviewer also gave me an option to code on my own laptop which I
had specifically carried there since the recruiter already told me about the
option of coding on whiteboard/laptop. But remember they don’t allow
using any code formatting and IDEs. I just had a basic editor to write code
with.

My Plan of Action For the Coding Interview?

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Just the same plan for the telephonic one but more extensive Leetcoding. I
remember I did Leetcode straight for 30 days for these coding rounds
around 3–4 hours a day. I used to solve as many medium level questions I
could with very little time spent on Hard level questions.

3. Onsite Coding Round 2:
Till this time I was in my Data Structure groove and ready for anything that

the interviewer was gonna throw at me. My state of mind being — “ What’s
the worst that could happen?”. And so I just carried on. The people at
Facebook were really nice as they asked for refreshments before and after
each interview and took care of not overextending any interview. There was
a lot of stock given to the fact that each interview starts at exactly the time
it needs to with a cool-off period of 15 mins between interviews.
Again some comparable(Not the same ones) problems in difficulty to
practice along with a brief description from Leetcode:

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a) API Based question Time-Based Key-Value Store: Create a key-value store
class, that supports two operations — set and get.
b) Merge k-Sorted Lists: You are given an array of

k

linked-lists

lists , each

linked-list is sorted in ascending order. Merge all the linked-lists into one
sorted linked-list and return it.
My goal in this coding interview was to be able to solve both the problems
that the interviewer put in a time period of 40 mins. But, this was a difficult
interview and I took most of the time of the interview in the second
problem which was of a hard level. Though the interviewer gave hints to

steer me towards the right data structure and algorithm. In the end, I was
able to solve problem 1 fully and most of problem 2.
A tip for interviewees would be to call out all solutions you have along with
the time complexity involved and only start writing code once you both
agree on a good solution.
Also, what I found out through these two interviews was that it was really
helpful to talk to your interviewers and explain your approach while you
are working. They sometimes provide hints and sometimes stop you from
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going to the wrong tangent. Even telling your interviewer where you are
stuck would help you as it sends out the signal to the interviewer as to
which direction you are thinking in. This also makes the whole interview
more collaborative and I think that is one of the qualities that the
interviewers are looking for in a person.

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Food is just awesome at FB. Though, I wasn’t able to enjoy much as I was a little anxious. Photo by Dan Gold on
Unsplash

Till this point, I was a little exhausted with all that whiteboard coding and
just the general interview pressure, and since it was around lunchtime, I
went to the Facebook cafeteria with an assigned colleague/buddy. This is

the part where you can ask about the company and this time is not rated on
the interviews so you can be quite open with questions about Facebook life
and such. And you can enjoy a wide variety of food in the Facebook
Cafeteria.

4. System Design

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Photo by Jeswin Thomas on Unsplash

This was another interview which I dreaded. As you can see, I was dreading
most of the interviews as it was quite an unnatural interview format for me.
In a system design interview, you are expected to create a service end to end
on a whiteboard. Some example problems for you to practice would be:
How would you design Netflix?
How would you design Youtube?
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How would you design Twitter/Facebook?
While this might look daunting, it is actually really open-ended when you
prepare for it. As in there are no wrong answers.
The way I like to go through this sort of interview is:
1. Design a very basic system that resembles the platform and has the

basic functionality the interviewer has asked for. For most of the
platforms, it would involve drawing boxes for server, client and a
database on a whiteboard.
2. Create a list of features I would like to have in the system. For example,
Follow in social networks, or booking a taxi on Uber, or double ticks in
Whatsapp when the message is read, or retweet functionality in Twitter,
or FB newsfeed, etc. The sky is the limit when it comes to features and
since we all have seen the features these platforms provide it should not
be that hard to come up with a feature list.
3. Add features to it throughout the interview and expand/change on the
very basic design. This might involve adding the feature as well as

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talking about scaling, handling edge cases, talking about the data
structures and databases involved, using caching etc.
4. Continue adding features and evolving the system till the end by
asking the interviewer what feature they would like to add based on a
list of features I provide them.

My Plan of Action For the System Design Interview?
There are pretty good resources on the internet to prepare for this interview
but I would like to mention two which I found very useful:
System Design Primer by Donne Martin: This is the resource that anyone
who prepares for system design should go through at least once but
honestly many times. The most important topics to learn here are
performance, scalability, latency, throughput, Availability, Consistency,

CDNs, Databases, Cache, Load balancing etc.
Youtube Videos of various system design on the most popular services: I
am talking about the big ones —
Netflix/Youtube/WhatsApp/Facebook/Gmail/Amazon etc. You can find
a lot of videos on youtube for system designs of all these services. The
one YouTuber I would definitely wanna call out would be Techdummies
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whose videos I watched for all these big platforms. And who really
explained concepts in the easiest way at least for me.
I spent a week where I just jumped from watching videos to reading the
repository by Donne Martin for this interview preparation back and forth
and I think that was just the right way. Also, it was fun to understand the
terms you would find a lot of engineers using, so it was a good learning
experience as well.
In the end, the most important thing in this interview is that you need to
drive the discussion with minimal input from the interviewer. Sometimes
the interviewer might request for a specific feature and you should
implement it but in the end, it is your system and you need to create and
add features you want in the most logical way to succeed in this round.

5. Behavioural
This interview tries to look at how you handle difficult situations. And, you
can prepare for this interview by assimilating and organizing all the work
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experiences you have had in the past, the problems you faced and solutions
you devised. You will need to collect all the instances where you diffused an
impossible situation or failed to deal with one.

Photo by Michelle Bonkosky on Unsplash

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All that said the best way to steer this interview is to Be Yourself!
For me, this interview was just like a discussion with the interviewer. He
started by giving his introduction and the work he was doing at FB. He then
asked me about the projects I was working on and we had a small
discussion about the ML parts of the projects. Then it was a very normal
discussion about how I would solve/behave in a situation like “What is a
mistake you made in your career you are not proud of?”(Not the exact
question I was asked). It is helpful if you sort of recollect all the good and
bad experiences and frame small stories while preparing for this interview.
But again the main point is being honest and a perfectly normal answer to a
question like — “Did you had a disagreement with your coworker?” could
be none if you have had zero disagreements. Be very honest about your
answers as the interviewers for these behavioural round can easily look
through someone if they lie.
As far as what I think, for me, this interview went pretty well.

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6. ML System Design

Source: Facebook ML Video Series

This was the interview which played to my strengths and honestly, I didn't
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prepare much for it. In this interview, I was expected to create a system to
solve an ML problem end to end.
In this interview, the interviewer is just assessing your ability to convert a
Business Problem to a Machine Learning system. So you might be given a
problem statement like develop a system to create a newsfeed using
Machine Learning, or create a system to filter out toxic comments or
honestly any Machine learning system.
You would then need to design a system end to end while talking about
various aspects of data and data collection, EDA, feature engineering,
model evaluation, model testing, putting the model in production and
finally maintenance and feedback.
A good resource to prepare for this interview is from Facebook itself:
Introducing the Facebook Field Guide to Machine Learning video series,
which is the only preparation I did for this interview.

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And that was a day!
And what a day it was. After the interview, I just went out roaming around
London to Trafalgar Square and just watched people performing various
tricks and antics. And then a walk back to my Hotel.
To end, it was just a nice interview experience and something I didn’t
expect would go well for me. Data Structures were the primary reason I
never used to apply for ML roles at big organizations but when I read about
them, I found them pretty doable and something you can learn if you can
put some time into them.
So, I am joining Facebook London just a year later now as a Software
Engineer(MLE) with my joining extended due to COVID related reasons.
Hoping this experience goes well.

Continue Learning
If you want to read up more on Algorithms and Data Structures in a more
structured way, here is an Algorithm Specialization on Coursera by
UCSanDiego. I sort of audited this course while preparing.

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Thanks for the read. I am going to be writing more beginner-friendly posts
in the future too. Follow me up at Medium or Subscribe to my blog to be
informed about them.

Also, a small disclaimer — There might be some affiliate links in this post to
relevant resources, as sharing knowledge is never a bad idea.

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