Cracking the Facebook Coding Interview

GayleL.McDowell | Founder/ CEO
gayle in/gaylemcdgayle
Cracking the Facebook
Coding Interview
I <3 Facebook!
CareerCup

gayle in/gaylemcdgayleGayle Laakmann McDowell 3
Why wouldFacebookprepyou?!?
Be more
comfortable
Be more prepared
Take out
the mystery
Avoid common
mistakes

Hi! I’m Gayle LaakmannMcDowell
Author Interview Coach Interview Consulting
<dev> </dev>
(CS) (MBA)

Gayle Laakmann McDowell 5gayle in/gaylemcdgayle
Yes! Slidesare online!
Gayle.com
 Click “Events”
 Ctrl-F for “Facebook”

A Typical**Process(**notuniversal!Askyourrecruiter)
Phone Onsite HiringCommittee & Decision
½ Behavioral
½ Algo / Coding
Design
Algo / Coding
Algo / Coding
Algo / Coding

gayle in/gaylemcdgayle 8
z
Gayle Laakmann McDowell
Typical
Coding
Interview
5 Minutes
Questions FORInterviewer
35 Minutes
Question #1 Question#2
5 Minutes
Prior Experience

z
Typical
Design
Interview
5 Minutes
Questions FORInterviewer
35 Minutes
Question#1
5 Minutes
Prior Experience

Behavioral
Questions
The soft squishy stuff.
02

The Pitch /Resume Walk-Through
• Showsof success
• Prompt the
interviewer
• Hobbies
I’masoftwareengineerat...
Mybackground’sinCS.
IstudiedatUPennandthen…
Atmycurrentcompany,I…
OutsideofworkI…

Your Past Work
3+ Projects
 Hard / cool
 You werecentral
 Technical depth
All Past Work
 TECHNICAL:Challenges,
architecture, tradeoffs,
successes, motivations
 SOFT:Teamwork, leadership,
conflicts, etc
What did YOU do?
What would you do differently?

z
What
about
YOU?
Be PASSIONATE
Be KNOWLEDGEABLE
Be a GOOD TEAMMATE

Design Questions
Big, meaty problems
03

z
How
To
Approach
W
W
Y
D
A
W
hat
ould
ou
o
t
ork

z
How
To
Approach
S
K
I
R
cope
eycomponents
dentify issues
epair

z
How
To
Approach
① Scope the Problem
 Askquestions
 Make appropriateassumptions
② Define Key Components
 Can besomewhatnaïve
③ Identify Issues
 Bottlenecks,tradeoffs
④ Repair & Redesign
Breadth-first,notdepth-first

ExampleQuestion

Design sketch
Frontend
Backend
Data store
logger

Collaborativediscussionthat you’re driving!
DRIVE
 Leadtheprocess
 Be openaboutissues
TEAMWORK
 Be opentofeedback
 Tweak asnecessary
Usethewhiteboard!

z
How
To
Prepare
Read about design of major companies
 THINK, don’t memorize!
Know key concepts
 Tasks, sharding, caches.
 Web stack, REST, etc
Practice back-of-the-envelope
calculations

Algorithm
Questions
Things that make you think
04

Why?
Analytical skills
How you think
Make tradeoffs
Pushthrough hard
problems
Communication
Strong CS fundamentals

Essential Knowledge
Data Structures Algorithms Concepts
ArrayLists Merge Sort BigO Time
Hash Tables QuickSort BigO Space
Trees(+Tries) & Graphs Breadth-FirstSearch Recursion
LinkedLists Depth-FirstSearch Memoization/ Dynamic
Programming
Stacks/ Queues BinarySearch
Heaps

Preparation
MASTER Big O
ImplementDS/Algorithms
Practicewith interview questions
Code on paper/whiteboard
Mock interviews
PUSHYOURSELF!

A Crash Course in Big O
Just getting your feet wet

Basicfor loop
Print0 throughN
Runtime?
 O(N)

Two loops
Printevens, thenodds
Runtime?O(N)

Basicfor loop
Printpairs from two arrays
Runtime?
 O(A*B)

Basicfor loop
Printordered pairs
Runtime?
 O(N2)
j : 0  N
i:0N
x x x x x x
x x x x x
x x x x
x x x
x x
x

Okay now thingsare getting tougher!

Okay now thingsare getting tougher!
Step 1: O(P)
 P = numberof people
Step 2: O(P * Y)
 Y = max life span
Step 3: O(L)
 L = last death year
O(P + P * Y + L)  O(P * Y + L)

Validate
Validate
Runtime?
 undefined

Fibonacci
Runtime?
Eeek. Recursion?

Fibonacci
Eeek. Recursion?
• Height of N
• Each level doubles # nodes
•  O(2N) time ***
•  O(N) space
Actuallyslightlylessforcomplexmathreasons.

Fibonacci, part 2
Runtime?
Eeek. Recursion?
• Height of N
• Each level has <=2nodes
•  O(N) time
•  O(N) space

Reminders
Drop constants
Don’t drop non-constants
Differentvariables
Avoid ‘n’
Add vs. Multiply
Recursion call tree

Solving Algorithms
How to… attempt to solve a hard problem

z
What
is NOT
expected
To know the answers
To solve immediately
To code perfectly
(It’snice.Itjustdoesn’t
happen*.)
*Okayfine.Ithappenedonce,in2000+hiringpackets.

z
What
IS
expected
Be excitedabout hard problems
Drive!
 Keep trying when stuck
 More than just “correct”
Pay attention to me!
Write real code
Showmehowyouthink!

z
How
To
Approach
CrackingTheCodingInterview.com“Resources”

gayle in/gaylemcdgayle 42Gayle Laakmann McDowell
step
Listen (for clues)

What’s the clue?
Anagram server
 Ex: rates ->aster, stare, taser, tears
Clue:why is it on a server?
Key(sorted string) Value (list of words)
aerst rates, aster, stare, taser, tears,
dgo dog, god
acll call
… …

step
Draw an Example
INTERSECTION SIZE: Find #
elementsin common between
two sorted, distinct arrays:

Ex:Intersection ofTwo Sorted Arrays
Most people draw somethinglike this:
[1, 12, 15, 19]
[2, 12, 13, 20]
 Toosmall
 Toospecial-case-y
• same size, one commonelement, same index

Ex:Intersection ofTwo Sorted Arrays
Better:
[1, 12, 15, 19, 20, 21]
[2, 15, 17, 19, 21, 25, 27]
 Big
 No specialcases

step
Draw an Example
Big Enough
General Purpose
+

step
Brute Force / Naive
Stupid&terribleisokay!

step
Optimize
Walk through brute
force
Look for optimizations

Techniquesto Develop Algorithms
BUD
Space and Time
Do It Yourself
Recursion

(A) Look for BUD
Bottlenecks
Unnecessary work
Duplicated work

What’s the bottleneck?
 Ex: countingthe intersection
[1, 12, 15, 19, 20, 21]
[2, 15, 17, 19, 21, 25, 27]
 Bottleneck:searching
B

What’s unnecessary?
 Ex: a3 + b3 = c3 + d3 (1 <=a,b, c, d<= 1000
 Unnecessary: looking for d
U

What’s duplicated?
 Ex: a3 + b3 = c3 + d3 (1 <=a,b, c, d<= 1000
 Duplicated: c, d pairs
D

 Ex: a3 + b3 = c3 + d3 (1 <=a,b, c, d<= 1000
D
c d c3 + d3
… … …
4 31 29855
4 32 32832
4 33 36001
… … …
5 59 205504
5 60 216125
5 61 227106
… … …

 Ex: a3 + b3 = c3 + d3 (1 <=a,b, c, d<= 1000
D
c3 + d3 (c, d)
… …
29855 (4, 31)
32832 (4, 32),(18, 30)
36001 (4, 33)
… …
205504 (5, 59)
216125 (5, 60),(45, 50)
227106 (5, 61)
… …

 Ex: a3 + b3 = c3 + d3 (1 <=a,b, c, d<= 1000
D

(B)Space/TimeTradeoffs
Hashtables & other datastructures
Precomputing

(C)Do it yourself
Findpermutationsof swithinb

find abbcd in
b a b c d b a e f d b b a c b d d f a e
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

(C)Do it yourself
Findpermutationsof swithinb
 s = abbcd
 b =
Findthem!
 … now how didyou actuallydoit?
b a b c d b a e f d b b a c b d d f a e

(D)Recursion
 Use, but don’t cling to, recursion
“instinct”
 Trybottom-up
 “Backtracking”
 Draw call-tree
 Derive runtime
 Find repeated subproblems
 Subsets of a set
 {} {}
 {a}{},{a}
 {a,b} {},{a},{b},{a,b}
 {a,b, c} …
 Subsets of {S1…Sn-1} +Sn to each

Techniquesto Develop Algorithms
BUD
Space and Time
Do It Yourself
Recursion

step
Walk Through
Know the variables
andwhen they change

step
Write Beautiful Code

How toWrite WhiteboardCode
Write straight
Top-leftcorner
Use arrows if needed
Error cases
Good style
Modularize (upfront!)
Languagechoiceisuptoyou!

Error Cases
Good to check errors / boundaries!
But consider the time…

Good Style
Spacing, line breaks, variable names

Language Choice
Java, Objective C, etc:Abbreviate
Python, Ruby, JS,etc: Carefulwithbuilt-infunctions

Modularization

Modularize(Upfront!)
I’ve learned
nothing.

step
Testing
FIRST Analyze
 What’s it doing? Why?
 Anything that looks weird?
 Errorhot spots
THEN use test cases
 Small test cases
 Edge cases
 Biggertest cases
BUT…
 Test code, notalgorithm
 Think as you test
 Think before you fix
Find permutationsof s withinb:
s = abbc
b = babcabbacaabcbabcacbb
for (i = 0; i < b.len – s.len; i++) {
if (isPerm(s, b.subs(i, s.len)) …
}

z
How
To
Approach
CrackingTheCodingInterview.com“Resources”

Questions for Your
Interviewer
What do you want to know?
05

Prepare some questions
What’s made you
happy / unhappy?
What are your goals?
Culture& work style
Career paths
Technology
Interviewer’s
experience

Final Thoughts
And questions
06

z
It’s done
for a
reason!
Be agreat teammate.
Be a great engineer.

Butwait, there’s onemore thing...
READY? Follow up with your recruiter to schedule your next steps.
JOB POSTINGS: facebook.com/careers/teams/engineering
SLIDES? Gayle.com -> Events -> search “Facebook”

Other Resources
Gayle.com
CareerCup.com
CrackingThe
CodingInterview.com
Or, follow me online
• facebook.com/gayle
• twitter.com/gayle
• gayle.com
• gayle@gayle.com
• quora.com

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