Cracking the Facebook Coding Interview I 3 Facebook

Cracking the Facebook Coding Interview I <3 Facebook! Gayle L. Mc. Dowell | Founder / CEO Career. Cup gayle in/gaylemcd

00 Why am I here?

Why would Facebook prep you? !? Be more comfortable Be more prepared be YOUR best Avoid common mistakes Gayle Laakmann Mc. Dowell Take out the mystery gayle in/gaylemcd 3

Hi! I’m Gayle Laakmann Mc. Dowell (CS) (MBA) <dev> </dev> Author Gayle Laakmann Mc. Dowell Interview Coach gayle in/gaylemcd Interview Consulting 4

Yes! Slides are online! Gayle. com Click “Events” Ctrl-F for “Facebook” m o c. yle a g @ g l: i a p e m r E p b f : t c Subje Gayle Laakmann Mc. Dowell gayle in/gaylemcd 5

m o c. e yl a g @ g : l i a p e m r E p b f : t c e Subj What to Expect

A Typical** Process (**not universal! Ask your recruiter) Hiring Committee & Decision Phone Onsite Algo / Coding ½ Behavioral ½ Algo / Coding Design Algo / Coding Gayle Laakmann Mc. Dowell gayle in/gaylemcd 7

5 Minutes Prior Experience Typical 45 Minutes Techz Question #1 Question #2 Interview 5 Minutes Questions FOR Interviewer Gayle Laakmann Mc. Dowell gayle in/gaylemcd 8

Email: [email protected] com Subject: fbprep 02 Behavioral Questions The soft squishy stuff.

The Pitch / Resume Walk-Through • Shows of success • Prompt the interviewer • Hobbies Gayle Laakmann Mc. Dowell gayle in/gaylemcd 10

Your Past Work 1 – 2 Hard Projects n n n All Past Work Hard / cool You were central Technical depth n n What did YOU do? Challenges, architecture, tradeoffs, mistakes, successes, motivations Teamwork, leadership, conflicts, etc What would you do differently? Gayle Laakmann Mc. Dowell gayle in/gaylemcd 11

Be PASSIONATE What about z YOU? Be KNOWLEDGEABLE Be a GOOD TEAMMATE Gayle Laakmann Mc. Dowell gayle in/gaylemcd 12

m o c. e l y a g @ Email: g p e r p b f : t c e j S ub 03 Design Questions Big, meaty problems

What Would You Do At Work How To z Approach Gayle Laakmann Mc. Dowell gayle in/gaylemcd 14

Scope Key components Identify issues Repair How To z Approach Gayle Laakmann Mc. Dowell gayle in/gaylemcd 15

① Scope the Problem Ask questions Make appropriate assumptions n n How To ② Define Key Components z Can be somewhat naïve n Approach ③ Identify Issues Bottlenecks, tradeoffs n ④ Repair & Redesign Discuss Top-> Down Gayle Laakmann Mc. Dowell gayle in/gaylemcd 16

How to Act DRIVE n n TEAMWORK Lead the process Be open about issues n n Be open to feedback Tweak as necessary Use the whiteboard! Gayle Laakmann Mc. Dowell gayle in/gaylemcd 17

Read about design of major companies n How To Know key concepts n z n Tasks, sharding, caches. Web stack, REST, etc Practice back-of-theenvelope calculations Prepare Gayle Laakmann Mc. Dowell THINK, don’t memorize! gayle in/gaylemcd 18

Ema il: g @ga Subj ect: f yle. c bpre om p 04 Algorithm Questions Things that make you think

Why? Analytical skills How you think Make tradeoffs Gayle Laakmann Mc. Dowell gayle Push through hard problems Communication Strong CS fundamentals gayle in/gaylemcd 20

Essential Knowledge Data Structures Algorithms Concepts Array. Lists Merge Sort Big O Time Hash Tables Quick Sort Big O Space Trees (+ Tries) Breadth-First Search Recursion Graphs Depth-First Search Memoization / Dynamic Programming Stacks / Queues Binary Search Heaps Gayle Laakmann Mc. Dowell gayle in/gaylemcd 21

Preparation MASTER Big O Implement DS/Algorithms Practice with interview questions Code on paper/whiteboard Mock interviews PUSH YOURSELF! Gayle Laakmann Mc. Dowell gayle in/gaylemcd 22

m o c. e yl a g @ g l: i a p e m r E p b f : t c Subje A Crash Course in Big O Just getting your feet wet

Basic for loop Print 0 through N Runtime? n Gayle Laakmann Mc. Dowell O(N) gayle in/gaylemcd 24

Nested loops Print pairs Runtime? n Gayle Laakmann Mc. Dowell O(N 2) gayle in/gaylemcd 25

More loops Print evens Runtime? n Gayle Laakmann Mc. Dowell O(N) gayle in/gaylemcd 26

Two loops Print evens, then odds Runtime? n Gayle Laakmann Mc. Dowell O(N) gayle in/gaylemcd 27

Basic for loop Print ordered pairs Runtime? O(N 2) x i: 0 N n j: 0 N x x x x x Gayle Laakmann Mc. Dowell gayle in/gaylemcd 28

Basic for loop Print ordered pairs Runtime? n Gayle Laakmann Mc. Dowell O(A * B) gayle in/gaylemcd 29

Okay now things are getting tougher! Step 1: O(P) n P = number of people Step 2: O(P * L) n L = max life span Step 3: O(Y) n Y = total # years O(P + P * L + Y) O(P * L + Y) Gayle Laakmann Mc. Dowell gayle in/gaylemcd 30

Eeek. Recursion? Fibonacci Runtime? • Height of K • Each level doubles # nodes • O(2 k) *** Actually slightly less for complex math reasons. Gayle Laakmann Mc. Dowell gayle in/gaylemcd 31

Eeek. Recursion? Fibonacci, part 2 Runtime? • Height of K • Each level has ONE node • O(k) Gayle Laakmann Mc. Dowell gayle in/gaylemcd 32

m o c. e yl a g @ g l: i a p e m r E p b f : t c Subje Solving Algorithms How to… attempt to solve a hard problem

To know the answers To solve immediately To code perfectly What is NOT z (It’s nice. It just doesn’t happen*. ) expected * Okay fine. It happened once, in 2000+ hiring packets. Gayle Laakmann Mc. Dowell gayle in/gaylemcd 34

Be excited about hard problems Drive! What IS z n Keep trying when stuck n More than just “correct” Pay attention to me! Write real code expected Show me how you think! Gayle Laakmann Mc. Dowell gayle in/gaylemcd 35

Cracking. The. Coding. Interview. com “Resources” How To z Approach m co. e l y a g @ g : l i Ema t: fbprep c e j b u S Gayle Laakmann Mc. Dowell gayle in/gaylemcd 36

Listen (for clues) step 1 Gayle Laakmann Mc. Dowell gayle in/gaylemcd 37

What’s the clue? Anagram server n Ex: rates -> aster, stare, taser, tears Clue: why is it on a server? Gayle Laakmann Mc. Dowell gayle in/gaylemcd 38

Draw an Example Big Enough step 2 Gayle Laakmann Mc. Dowell + General Purpose gayle in/gaylemcd 39

Ex: Intersection of Two Sorted Arrays Most people draw something like this: [1, 12, 15, 19] [2, 13, 20] n n Too small Too special-case-y • same size, one common element, same index Gayle Laakmann Mc. Dowell gayle in/gaylemcd 40

Ex: Intersection of Two Sorted Arrays Better: [1, 12, 15, 19, 20, 21] [2, 15, 17, 19, 21, 25, 27] n n Gayle Laakmann Mc. Dowell Big No special cases gayle in/gaylemcd 41

Brute Force / Naive step 3 Stupid & terrible is okay! Gayle Laakmann Mc. Dowell gayle in/gaylemcd 42

Optimize step 4 Gayle Laakmann Mc. Dowell Walk through brute force Look for optimizations gayle in/gaylemcd 43

Techniques to Develop Algorithms BUD Space and Time Do It Yourself Recursion Gayle Laakmann Mc. Dowell gayle o y h s u P gayle in/gaylemcd ! f l e s r u 44

(A) Look for BUD Bottlenecks Unnecessary work Duplicated work Gayle Laakmann Mc. Dowell gayle in/gaylemcd 45

What’s the bottleneck? Ex: counting the intersection [1, 12, 15, 19, 20, 21] [2, 15, 17, 19, 21, 25, 27] Bottleneck: searching B Gayle Laakmann Mc. Dowell gayle in/gaylemcd 46

What’s unnecessary? Ex: a 3 + b 3 = c 3 + d 3 (1 <= a, b, c, d <= 1000 Unnecessary: looking for d U Gayle Laakmann Mc. Dowell gayle in/gaylemcd 47

What’s unnecessary? Ex: a 3 + b 3 = c 3 + d 3 (1 <= a, b, c, d <= 1000 Unnecessary: looking for d U Gayle Laakmann Mc. Dowell gayle in/gaylemcd 48

What’s duplicated? Ex: a 3 + b 3 = c 3 + d 3 (1 <= a, b, c, d <= 1000 Duplicated: c, d pairs Gayle Laakmann Mc. Dowell gayle D in/gaylemcd 49

What’s duplicated? Ex: a 3 + b 3 = c 3 + d 3 Duplicated: c, d pairs Gayle Laakmann Mc. Dowell gayle (1 <= a, b, c, d <= 1000 c d c 3 + d 3 … … … 4 31 29855 4 32 32832 4 33 36001 … … … 5 59 205504 5 60 216125 5 61 227106 … … … in/gaylemcd D 50

What’s duplicated? Ex: a 3 + b 3 = c 3 + d 3 (1 <= a, b, c, d <= 1000 c 3 + d 3 (c, d) … … 29855 (4, 31) 32832 (4, 32), (18, 30) 36001 (4, 33) … … 205504 Duplicated: c, d pairs 216125 (5, 59) 227106 (5, 61) … … Gayle Laakmann Mc. Dowell gayle (5, 60), (45, 50) in/gaylemcd D 51

What’s duplicated? Ex: a 3 + b 3 = c 3 + d 3 (1 <= a, b, c, d <= 1000 D Gayle Laakmann Mc. Dowell gayle in/gaylemcd 52

What’s duplicated? Ex: a 3 + b 3 = c 3 + d 3 (1 <= a, b, c, d <= 1000 D Gayle Laakmann Mc. Dowell gayle in/gaylemcd 53

(B) Space/Time Tradeoffs Hash tables & other data structures Precomputing Gayle Laakmann Mc. Dowell gayle in/gaylemcd 54

Space/Time Tradeoffs Precomputing Find rectangle at origin w biggest sum 6 5 -9 2 -2 -5 -2 7 3 -2 10 13 -8 -3 1 -2 Brute force: compute all rectangles and sums Gayle Laakmann Mc. Dowell gayle in/gaylemcd 55

Space/Time Tradeoffs Precomputing Find rectangle with biggest sum 6 5 -9 2 -2 -5 -2 7 3 -2 10 13 -8 -3 1 -2 = Gayle Laakmann Mc. Dowell - + gayle + 10 in/gaylemcd 56

Space/Time Tradeoffs Precomputing Find rectangle with biggest sum 6 5 -9 2 -2 -5 -2 7 3 -2 10 13 -8 -3 1 -2 = Gayle Laakmann Mc. Dowell - + gayle + 13 in/gaylemcd 57

(C) Do it yourself Find permutations of s within b s = abbc n b = babcabbacaabcbabcacbb n Find them! n Gayle Laakmann Mc. Dowell … now how did you actually do it? gayle in/gaylemcd 58

(D) Recursion / Base Case & Build Subsets of a set n n {} {a} {}, {a} {a, b} {}, {a}, {b}, {a, b} {a, b, c} … Subsets of {S 1…Sn-1} + Sn to each Gayle Laakmann Mc. Dowell gayle • Careful! “This sounds recursive” is 50% accurate • Bottom-up approach, topdown implementation • “Backtracking” Recursion • Draw a “call tree”. Look for repeated subproblems. in/gaylemcd 59

Techniques to Develop Algorithms BUD Space and Time Do It Yourself Recursion Gayle Laakmann Mc. Dowell gayle o y h s u P gayle in/gaylemcd ! f l e s r u 60

Walk Through step 5 Gayle Laakmann Mc. Dowell Know the variables and when they change gayle in/gaylemcd 61

Write Beautiful Code step 6 Gayle Laakmann Mc. Dowell gayle in/gaylemcd 62

How to Write Whiteboard Code Write straight Top-left corner Use arrows if needed Error cases / TODOs Good variables Modularized Language choice is up to you! Gayle Laakmann Mc. Dowell gayle in/gaylemcd 63

Modularization Gayle Laakmann Mc. Dowell gayle in/gaylemcd 64

Testing FIRST Analyze step n 7 n n THEN use test cases n n n Small test cases Edge cases Bigger test cases BUT… n n Gayle Laakmann Mc. Dowell What’s it doing? Why? Anything that looks weird? Error hot spots gayle Test code, not algorithm Think before you fix gayle in/gaylemcd 65

m o c. e yl a g @ g : l Emai t: fbprep c e j b Su 05 Questions for Your Interviewer What do you want to know?

Be Prepared With Questions Passion Knowledge Interest Communication Curiosity Gayle Laakmann Mc. Dowell gayle in/gaylemcd 67

Consider… What’s made you happy / unhappy? What are your goals? Gayle Laakmann Mc. Dowell gayle Culture & work style Career goals Technology gayle in/gaylemcd 68

Ema il: Sub ject: [email protected] le. c fbpr om ep 06 Final Thoughts And questions

Be a great teammate. It’s done for az reason! Gayle Laakmann Mc. Dowell Be a great engineer. gayle in/gaylemcd 70

But wait, there’s one more thing. . . m o c. e yl a g @ g : l i a p e m r E p b f : t c e Subj SURVEY: Coming your way. READY? Follow up with your recruiter to schedule your next steps. JOB POSTINGS: facebook. com/careers/teams/engineering SLIDES? Gayle. com -> Events -> search “Facebook” Gayle Laakmann Mc. Dowell gayle in/gaylemcd 71

Other Resources Gayle. com Career. Cup. com Cracking. The Coding. Interview. com Or, follow me online • • • m o c. e l y a g Email: [email protected] ep r p b f : t c e j b Su Gayle Laakmann Mc. Dowell gayle in/gaylemcd facebook. com/gayle twitter. com/gayle. com [email protected] com quora. com 72