Cracking the Coding Interview (Oct 2012)

Cracking the Coding Interview
Advice and Strategies for Software
Engineering Interviews
McDowell | CareerCup.com
July 2013
Gayle Laakmann McDowell
Founder / CEO, CareerCup.com
Author of The Google Resume and Cracking the Coding Interview

TechnicalSkillsBehavioralApplyingProcess
My Background
• Software Engineer @ Google, Microsoft
& Apple
– 3 Years on Google Hiring Committee
– Interviewed 150+ candidates
• Founder of CareerCup.com
• Author
– Cracking the Coding Interview
– The Google Resume

What to Expect
• 5 - 10 min – Qs about resume
• 40 min – technical Qs
– Coding and algorithms
• 5 – 10 min – let you ask Qs, general
wrap up, etc

TechnicalSkillsBehavioralApplyingProcess Big Company Evaluation (Devs)
Intelligence
Coding
Skills
Personality
Experience
Testing
aptitude, not
knowledge

Start-Ups
• Coding + Intelligence
• “Hit the ground running”
– Do you know “their” technologies?
• Entrepreneurial
– Have you started things?
– How much direction do you need?
• Personality
– Will you fit with the team?
but…
it varies!

PM Roles
• Communication Skills
• User-Focused Thinking
• Passion for Technology
• Analytical Skills
• Technical Skills (position dependent)

How You Are Judged
How did you do RELATIVE
to other candidates on
the SAME question?
… it’s about how quickly
you solved it relative to
other candidates.
It’s not about how
quickly you solved
the problem…

Resumes & Application Process

How to Get an Interview
• Your “Pedigree”
– Education
– Work Experience
– Projects
– Github / Online Profile
• Make a kick-ass resume

How We Review Resumes
1. Pull resume out of giant stack
2. Spot-check: company
names, positions, projects, schoo
ls.
3. Skim bullets to see if you’ve
written real code.
InterviewReject
4. Go to next resume & whine about
how many more you have left.
“Glanced
at,” not read.
15 – 30
seconds

How CS Resume
Should Look
Short (1 – 2 line bullets)
3 – 4 Projects
Courses & independent
Finished or unfinished
List of Technical Skills
Short! Cut the “fluff.”
GPA if at least 3.0
max (in-major, overall)
One Page Only!
Unless > 10 years exp.
A Real Resume Format
with organized columns
Focus on Accomplishments
not responsibilities
Observe: No Objective!
Objectives / summaries are
almost always useless.

Talking to Recruiters
• Show:
– What you’ve done (mobile app? Show it!)
– What you’re excited about
• Have a “pitch” ready
• Weird is okay
– (arrogant is not)

Soft Questions

Goals of Your Answer
• Answer the question.
• Deliver a good answer.
• Communicate well.
– Nugget First
– S.A.R.: Situation, Action, Result

Preparing for Behavioral Qs
• Create Preparation Grid for Projects
OS Project Amazon Intern.
Enjoyed
Hated
Most Challenging
Hardest Bug
+ Behavioral Grid [for PM & less tech. roles]

Structure 1: Nugget First
• Lead with your “thesis” / nugget
– Grabs the listener’s attention
– Gives them context for where you’re going.
A: I’m most proud of the way I
re-architected the …
Q: What accomplishment are you
most proud of?

Structure 2: S.A.R.
ituation
ction
esult
S
A
R
What was the issue?
What did you do about it?
What was the impact?

TechnicalSkillsBehavioralApplyingProcess Good Questions for Interviewer
• Qs you want to get answered
– How decisions get made
– Interaction betw devs, PMs, testers
• Qs that show interest / passion
– What’s the architecture like?
• Qs that show knowledge / aptitude
– How did you solve problem with ___?
Don’t worry too much
about this stuff

Technical Skills
Interview Prep

How to study
• Study the basics
– Complex algorithms generally unnecessary.
• Practice solving questions
– Don’t memorize!
– See: CtCI & CareerCup.com
• Push yourself!
• Write code on paper

Data Structures
• How to implement
• When to use (pros / cons)
Linked Lists Stacks Queues
Trees Tries Graphs
Vectors Heaps Hashtables

Algorithms
• Implementation
• Space vs. Time
Complexity
Quick Sort Merge Sort
Tree Insert / Find Binary Search
Breadth-First
Search
Depth-First
Search

Concepts
• Not just a concept – know how to code!
Threading
Big-O
Time / Space
Memory
Management
Recursion
Probability +
Combinatorics
Bit Manipulation

TechnicalSkillsBehavioralApplyingProcess How to Learn CS Fundamentals?
• Necessary for “elite” tech companies
– Not necessarily otherwise (check interview
questions)
• MIT Open Courseware
– Freshman / sophomore level DS & Algo
courses
• Books
– CLRS (Algorithms)
• Online tutorials
• Practice implementing and using

Technical Skills
Mastering the Interview

Types of “Serious” Questions
1. Product Design Questions
2. Estimation Questions
3. Software Engineering Questions
– Coding & Algorithms
– Object Oriented Design
– Scalability
– Factual / Trivia / Language-Based

Product Design Questions
How would you design an
calculator for the blind?
Design an elevator for a building.
Pick a Google product.
How would you improve it?

TechnicalSkillsBehavioralApplyingProcess Product Design Questions: Why?
• Communication & Structured Thinking
• Ability to understand the user
• Creativity
• Business instincts / skills

Product Design Qs: Approach
1. Ask questions to resolve ambiguity
2. Understand the user
3. Structure the problem
4. Solve piece by piece

Product Design Qs: Example
Step 1: Ask Questions
• Adults? Children? Professionals?
• Where are they using it?
– School, work, etc.
How would you design a calculator for the blind?

Step 2: Understand the User
• What’s important to a blind child?
– Keeping up with the rest of the class
– Not feeling “different”
– Efficient input / output
• What about
teachers, parents, classmates, etc.?

Step 3: Structure
• Find a structure
– Otherwise, you’re just blabbering
• One approach:
1. Make list of functions necessary
2. Discuss how to do input / output
3. Usability for non-blind
4. Summary

Step 4: Solve!

Estimation Questions
How many tennis balls
can fit in an SUV?
How much money does Gmail make
from ads every year?
How much do New Yorkers
spend on electricity each year?

Estimation Qs: Why?
• Problem Solving
• Basic Quantitative Skills

TechnicalSkillsBehavioralApplyingProcess Estimation Qs: How to Approach
1. Ask questions to resolve ambiguity
– Don’t make assumptions (yet)
2. Outline / Structure Your Approach
3. Break down the components
– Assume numbers when necessary
– State assumptions explicitly
– Round numbers to make your math easier
4. Sanity Check
– Do your numbers make sense?

Estimation Qs: Example
Step 1: Ambiguous Information
• Profit or revenue?
• Past year? Or average over history?
• Gmail only? Or include Google Apps?
How much money does Gmail make from ads every year?

Step 2: Outline Your Approach
(# of users)
x
(# clicks / year)
x
( $ / click )

Step 3: Break down components
Estimate # of Gmail users in the US
1. Assume 300 million people in the US.
– Exclude 0 - 12 years old and 65 - 75 years old
–  ~ 200 million
2. Assume 80% of people use email
3. 80% of those have non-work account
… and so on …

Step 4: Validate Numbers
• Could revenue be $5 billion?
• No, because…
– Google’s annual revenue is ~$40 billion
– $16 / US citizen (not just Gmail users)

TechnicalEstimationProductDesign
CS Questions
• Knowledge Questions
• Coding & Algorithms
• Object Oriented Design
• Scalability

Knowledge & Trivia Questions
• Not key focus at “elite” tech companies.
– If asked, tends to focus more on DS
knowledge than language knowledge
• If you don’t know, try to deduce the
answer
– How would language designer do it? etc

Coding & Algorithm Questions
1. Ask Questions!
– Questions are more ambiguous than they appear
2. Talk out loud
– Show us how you think
3. Think critically
– Does your algorithm really work? What’s the
space and time complexity?
4. Code slowly and methodically
– It’s not a race
5. Test your code
– And make CAREFUL fixes.
If you’ve heard a
question before,
admit it.

What does a “good coder” do?
• Be methodical. Don’t try to rush.
• Reasonably Bug Free
– Thorough testing (and careful fixing)
– Check for error conditions
• Clean coding
– Use other functions
– Good use of data structures (define own if useful)
– Concise and readable

Whiteboard Coding Tips
• Code in the FAR top left corner
• Write small & straight
• Pseudocode if you’d like
• Use arrows to insert new code snippets
– Erasing wastes time
• Use “good” variable names, then
abbreviate
• “Breadth-First” Coding

“Breath-First” Coding
• Divide problems into sub-components
– Modularize & Structure
• Ex: Ransom Note
– Write as one big function?
– OR… modularize!

Ex: Ransom Note:
“Depth-First” Coding

Ex: Ransom Note:
“Breadth-First” Coding

“Breath-First” Coding
• “Depth-First” Coding (One big function)
– Messy
– Time consuming
– Poor coding style
• “Breadth-First” Coding (Modularized)
– “Finishes” algorithm immediately
– Often allows you to skip details
– Easier to test & fix issues
– Demonstrates good coding style
– Keeps YOU organized

Common Mistakes
• Ignoring information given
• Not using examples
– To design algorithms
– While writing code
• “Pushing through” code when confused
– Incoherent thoughts -> incoherent code
• Diving into code too quickly
• The details:
– Off-by-one, < vs <=, division (and any math)

TechnicalEstimationProductDesign Types of Interview Questions
Coding & Algorithms Object Oriented Design System Design
“Reverse a Linked
List”
“Design a Parking
Lot”
“Design a
Web Crawler”

TechnicalEstimationProductDesign Types of Interview Questions
Coding & Algorithms
Pattern Matching
Simplify & Generalize
Base Case & Build
Data Structure
Brainstorm
Object Oriented Design System Design

TechnicalEstimationProductDesign Algorithm Qs: Pattern Matching
Q: Write code to reverse the order of words
in a sentence.
“dogs are cute”
“cute are dogs”
Similar to: reverse characters in a string.
“dogs are cute”
“etuc era sgod”
A: Reverse full string, then reverse each
word.
Coding & Algorithms
Pattern Matching
Base Case & Build
Data Structure
Brainstorm

TechnicalEstimationProductDesign Algorithm Qs: Simplify & Generalize
Coding & Algorithms
Pattern Matching
Base Case & Build
Data Structure
Brainstorm
Q: Design algorithm to figure out if you can
build a ransom note (array of strings) from a
magazine (array of strings).
Simplify: what if we used characters instead
of strings?
 Build array of character frequencies.
Generalize: how we can extend answer to
words?
A: Build hashtable from word to frequency.

TechnicalEstimationProductDesign Algorithm Qs: Base Case & Build
Coding & Algorithms
Pattern Matching
Base Case & Build
Data Structure
Brainstorm
Q: Design algorithm to print subsets of set.
{a, b, c}  {}, {a}, {b}, {c}, {a, b},
{a, c}, {b, c}, {a, b, c}
S({})  {}
S({a})  {}, {a}
S({a, b})  {}, {a}, {b}, {a, b}
S({a, b, c})  ?
A: Build S(n) by cloning S(n-1) and adding n
to the cloned sets.

TechnicalEstimationProductDesign Algorithm Qs: Data Structure Brainstorm
Coding & Algorithms
Pattern Matching
Base Case & Build
Data Structure
Brainstorm
Q: There are 10^10 possible phone #s.
Explain how you could efficiently implement
assignSpecificNum(num) and
assignAnyAvailableNum().
Array (sorted)? Too slow to remove num.
Linked list? Too slow to find specific num.
Hash table? Can’t iterate through free nums.
Tree? Ah-ha!
A: Store free #s in BST. Remove when taken.

Coding & Algorithms
Pattern Matching
Base Case & Build
Data Structure
Brainstorm
How To Solve Algorithm Questions
Compare to similar problems.
Solve first for a simplified / tweaked problem.
Solve for n = 1, and build solution for n = 2.
Try to apply data structure to solve problem.

TechnicalEstimationProductDesign Object Oriented Design
Handle Ambiguity
What about the question is ambiguous?
Design the Core Objects
What are the main objects in the system?
Analyze Relationships
How are the objects related to each other?
Investigate Actions
What are the main operations?

Object Oriented Design
Handle Ambiguity
Is it a single restaurant, or part of a chain?
Design the Core Objects
Guest, Party, Table, Server, Host, …
Analyze Relationships
Server and Host are both Employees…
Investigate Actions
A Party is seated at a Table by a Host…
How would you
design the data
structures and
objects for a
restaurant?

TechnicalEstimationProductDesign System Design
Handle Ambiguity
What about the question is ambiguous?
Make Believe
Pretend there wasn’t so much data & solve
Get Real
Go back to the real problem. What breaks?
Solve Problems
Solve the issues you just found.

TechnicalEstimationProductDesign System Design
Handle Ambiguity
Do the words need to be in a specific
order?
Make Believe
Assume everything can fit on one machine.
Get Real
Must split up data across machines.
Solve Problems
Divide hash table by file or by keyword?
Given millions of
documents, find
all documents
which contain a
list of words.

TechnicalEstimationProductDesign Whew! All Done!
<Gulp> This is a lot of
stuff. Do I need to get
everything right?
Coding & Algorithms
Pattern Matching
Base Case & Build
Data Structure
Brainstorm

<Gulp> This is a lot of
stuff. Do I need to get
everything right?
</Gulp>
Evaluation is RELATIVE,
not absolute.
… it’s about how quickly
you solved it relative to
other candidates.
It’s not about how
quickly you solved
the problem…

So RELAX!
Interviews are supposed to be hard!
Everyone makes mistakes.
Everyone!

After Your Interview
• Follow-up with your recruiter
– No response != rejection
• You have no idea how well/poorly you
did.
– Seriously. I know you think you do. But you
don’t.
• Lots of randomness.
– So if you fail, get up and try again.

Other Resources
CareerCup.com
• Interview Videos
• iPhone App
• Resume Review
• Mock Interviews
Or, follow me online at…
• twitter.com/gayle
• facebook.com/gayle
• technologywoman.com
• gayle@careercup.com

Cracking the Coding Interview (Oct 2012)

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