cracking the coding interview (oct 2012)

Cracking the Coding Interview

Advice and Strategies for Software Engineering Interviews

McDowell | CareerCup.comJuly 2013

Gayle Laakmann McDowellFounder / CEO, CareerCup.com

Author of The Google Resume and Cracking the Coding Interview

McDowell | CareerCup.com

Technical SkillsBehavioralApplying

Process

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



Process

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



Process

Big Company Evaluation (Devs)

IntelligenceCodingSkills

PersonalityExperience

Testing aptitude, not knowledge



Process

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!



Process

PM Roles

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



Process

How You Are Judged

How did you do RELATIVEto 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




Process

How to Get an Interview

• Your “Pedigree”– Education– Work Experience– Projects– Github / Online Profile

• Make a kick-ass resume



Process

How We Review Resumes

1. Pull resume out of giant stack2. Spot-check: company names,

positions, projects, schools.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 ProjectsCourses & independentFinished or unfinished

List of Technical SkillsShort! Cut the “fluff.”

GPA if at least 3.0max (in-major, overall)

One Page Only!Unless > 10 years exp.

A Real Resume Formatwith organized columns

Focus on Accomplishmentsnot responsibilities

Observe: No Objective!Objectives / summaries are

almost always useless.



Process

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




Process

Goals of Your Answer

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



Process

Preparing for Behavioral Qs

• Create Preparation Grid for ProjectsOS Project Amazon Intern.

Enjoyed

Hated

Most Challenging

Hardest Bug

+ Behavioral Grid [for PM & less tech. roles]



Process

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?



Process

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?



Process

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 SkillsInterview Prep




Process

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



Process

Data Structures

• How to implement• When to use (pros / cons)

Linked Lists Stacks Queues

Trees Tries Graphs

Vectors Heaps Hashtables



Process

Algorithms

• Implementation• Space vs. Time

ComplexityQuick Sort Merge Sort

Tree Insert / Find Binary Search

Breadth-First Search

Depth-First Search



Process

Concepts

• Not just a concept – know how to code!

Threading Big-O Time / Space

MemoryManagement

Recursion Probability + Combinatorics Bit Manipulation



Process

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 SkillsMastering the Interview




Process

Types of “Serious” Questions

1. Product Design Questions 2. Estimation Questions3. Software Engineering Questions– Coding & Algorithms– Object Oriented Design– Scalability– Factual / Trivia / Language-Based


Process

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?


Process

Product Design Questions: Why?

• Communication & Structured Thinking• Ability to understand the user• Creativity• Business instincts / skills


Process

Product Design Qs: Approach

1. Ask questions to resolve ambiguity2. Understand the user3. Structure the problem4. Solve piece by piece


Process

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?


Process


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.?



Process


Step 3: Structure• Find a structure– Otherwise, you’re just blabbering

• One approach:1. Make list of functions necessary2. Discuss how to do input / output3. Usability for non-blind4. Summary



Process


Step 4: Solve!



Process

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?


Process

Estimation Qs: Why?

• Problem Solving

• Basic Quantitative Skills


Process

Estimation Qs: How to Approach

1. Ask questions to resolve ambiguity– Don’t make assumptions (yet)

2. Outline / Structure Your Approach3. 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?


Process

Estimation Qs: Example

Step 1: Ambiguous Information• Profit or revenue?• Past year? Or average over history?• Gmail only? Or include Google Apps?



Process


Step 2: Outline Your Approach(# of users)

x(# clicks / year)

x( $ / click )



Process


Step 3: Break down componentsEstimate # 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 email3. 80% of those have non-work account… and so on …



Process


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)



Technical

Estimation

Product Design

CS Questions

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


Technical

Estimation

Product Design

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


Technical

Estimation

Product Design

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.


Technical

Estimation

Product Design

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


Technical

Estimation

Product Design

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


Technical

Estimation

Product Design

“Breath-First” Coding

• Divide problems into sub-components– Modularize & Structure

• Ex: Ransom Note– Write as one big function?– OR… modularize!


Technical

Estimation

Product Design

Ex: Ransom Note: “Depth-First” Coding


Technical

Estimation

Product Design

Ex: Ransom Note: “Breadth-First” Coding


Technical

Estimation

Product Design

“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


Technical

Estimation

Product Design

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)


Technical

Estimation

Product Design

Types of Interview Questions

Coding & Algorithms

Object Oriented Design System Design

“Reverse a Linked List”

“Design a Parking Lot”

“Design a Web Crawler”


Technical

Estimation

Product Design

Types of Interview QuestionsCoding & Algorithms

Pattern Matching

Simplify & Generalize

Base Case & Build

Data Structure Brainstorm

Object Orient

ed Design

System

Design


Technical

Estimation

Product Design

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


Object Orient

ed Design

System

Design


Technical

Estimation

Product Design

Algorithm Qs: Simplify & GeneralizeCoding & Algorithms

Pattern Matching


Base Case & Build


Object Orient

ed Design

System

Design

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.


Technical

Estimation

Product Design

Algorithm Qs: Base Case & BuildCoding & Algorithms

Pattern Matching


Base Case & Build


Object Orient

ed Design

System

Design

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.


Technical

Estimation

Product Design

Algorithm Qs: Data Structure BrainstormCoding & Algorithms

Pattern Matching


Base Case & Build


Object Orient

ed Design

System

Design

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.


Technical

Estimation

Product Design

Coding & Algorithms

Pattern Matching


Base Case & Build


Object Orient

ed Design

System

Design

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.


Technical

Estimation

Product Design

Object Oriented Design

Handle AmbiguityWhat about the question is ambiguous?

Design the Core ObjectsWhat are the main objects in the system?

Analyze RelationshipsHow are the objects related to each other?

Investigate ActionsWhat are the main operations?

Coding & Algorithms



Technical

Estimation

Product Design

Coding & Algorithms


Object Oriented Design

Handle AmbiguityIs it a single restaurant, or part of a chain?

Design the Core ObjectsGuest, Party, Table, Server, Host, …

Analyze RelationshipsServer and Host are both Employees…

Investigate ActionsA Party is seated at a Table by a Host…

How would you design the data structures and objects for a restaurant?


Technical

Estimation

Product Design

System Design

Handle AmbiguityWhat about the question is ambiguous?

Make BelievePretend there wasn’t so much data & solve

Get RealGo back to the real problem. What breaks?

Solve ProblemsSolve the issues you just found.

Coding & Algorithms



Technical

Estimation

Product Design

System Design

Handle AmbiguityDo the words need to be in a specific order?

Make BelieveAssume everything can fit on one machine.

Get RealMust split up data across machines.

Solve ProblemsDivide hash table by file or by keyword?

Given millions of documents, find all documents

which contain a list of words.

Coding & Algorithms



Technical

Estimation

Product Design

Whew! All Done!

<Gulp> This is a lot of stuff. Do I need to get

everything right?

Coding & Algorithms

Pattern Matching


Base Case & Build


Object Orient

ed Design

System

Design



Process

<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…



Process

So RELAX!Interviews are supposed to be hard!

Everyone makes mistakes. Everyone!



Process

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.



Process

Other ResourcesCareerCup.com• Interview Videos• iPhone App• Resume Review• Mock Interviews

Or, follow me online at…• twitter.com/gayle• facebook.com/gayle• technologywoman.com• [email protected]