Cracking the Algorithm & Coding Interview

GayleL.McDowell | Founder/ CEO
gayle in/gaylemcdgayle
Cracking the Algorithm &
Coding Interview
SVCC
SiliconValleyCodeCamp

gayle in/gaylemcdgayleGayle Laakmann McDowell 2
Hi! I’m Gayle LaakmannMcDowell
Author Interview Coach Interview Consulting
<dev> </dev>
(CS) (MBA)

Gayle Laakmann McDowell 3gayle in/gaylemcdgayle
Yes! Slidesare online!
Gayle.com
 Click“Events”
 Ctrl-F for “SiliconValley”

But why?
Why why why why why why meeeee
01

gayle in/gaylemcdgayle 5
z
Gayle Laakmann McDowell
What
Really
Happens

Why?
Strong CS
fundamentals
Analytical skills
Make tradeoffs
Push throughhard
problems
Communication
How you think

Preparation
02

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

Preparation
ImplementDS/Algorithms
MASTER BigO
Practice with interviewquestions
Code on paper/whiteboard
Mock interviews
PUSHYOURSELF!

Expectations &
Evaluation
03

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
More thanjust “correct”
Drive!
Keeptrying when stuck
Write real code
Showmehowyouthink!

z
How
You’re
Evaluated
RELATIVE
Not a “metric” / timer
Nooneisperfect!

Solving
04

z
How
To
Approach
CrackingTheCodingInterview.com“Resources”

gayle in/gaylemcdgayle 16Gayle Laakmann McDowell
step
Listen (for clues)

What’sthe clue?
Anagram server
 Ex: rates ->aster, stare, taser, tears
Clue:why is it ona server?

step
Draw an Example
Big Enough
General Purpose
+

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 common element, sameindex

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

step
Brute Force / Naive
Stupid&terribleisokay!

step
Optimize
Walk through brute
force
Look for optimizations

Techniques to Develop Algorithms
Optimize
A. BUD
B. Space/time
C. Doityourself
Solve
D. Recursion
E. Solve “incorrectly”
F. Other data structures
Pushyourself!

(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
 Duplicated: c, d pairs
D

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

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

Space/Time Tradeoffs Hashtables
 Find # pairs withsum
INPUT: array = [5, 15, 8, 9, 3, 2, -1, 4]
sum = 7
OUTPUT: 3
pairs = (5, 2), (8, -1), (3, 4)
 Putitemsinto hashtable

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 rectanglesand sums

 Find rectangle with biggest sum
6 5 -9 2
-2 -5 -2 7
3 -2 10 13
-8 -3 1 -2
-+ + 10=

 Find rectangle with biggest sum
6 5 -9 2
-2 -5 -2 7
3 -2 10 13
-8 -3 1 -2
-+ + 13=

(C)Do it yourself
Findpermutationsof swithinb
 s = abbc
 b = babcabbacaabcbabcacbb
Findthem!
 … now how didyou actuallydoit?

Techniques to Develop Algorithms
Optimize
A. BUD
B. Space/time
C. Doityourself
Solve
D. Recursion
E. Solve “incorrectly”
F. Other data structures
Pushyourself!

(D) Recursion/ Base Case & Build
Subsets of a set
 {}  {}
 {a}  {}, {a}
 {a, b}  {}, {a}, {b}, {a, b}
 {a, b, c}  …
Subsets of {S1…Sn-1} + Sn to each

(E) Solve “incorrectly”
① Develop incorrectsolution
② Identifywhy preciselyit’s incorrect
③ Repair
④ (& Repeat)

(E)Solve “incorrectly”
Random node in BST
Try: flipcoin
Coin=Heads
 Branch Left
Coin=Tails
 Branch Right

Try: random number in {0, 1, 2}
R=0
 Branch Left
R=2
 Branch Right
R=1
 Return root

Try:
 Return rootwith1/n probability
 Then flipcoin(heads left,tails->right)

Try: pick random # 0 throughn-1
R=0
 Return root
R>left.size
 Branch right
1<=R<=left.size
 Branch left

(F) Other Data Structures
Giving outphone numbers
 “I wantany availablenumber”
 “I wantthisnumber”
Try: sorted array?Sorted linkedlist?Hashtable?
BST?

step
Walk Through
Know the variables
andwhen they change

step
Write Beautiful Code

How to Write WhiteboardCode
Modularized
Error cases / TODOs
Good variables
Write straight
Top-leftcorner
Use arrows if needed
Languagechoiceisuptoyou!

Modularization

step
Testing
FIRST Analyze
 What’sitdoing?Why?
 Anythingthatlooksweird?
 Errorhotspots
THEN use test cases
 Smalltestcases
 Edgecases
 Bigger testcases

Final Thoughts
And questions
05

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

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

Cracking the Algorithm & Coding Interview

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