Google Hash Code 2017

UniBG - 08 / 02 / 2017seclab.unibg.it

http://seclab.unibg.it/http://seclab.unibg.it/

Google Hash Code

● What is it?

● How does it work?○ Team composed by 2-4 members

○ It’s necessary to register the team and then join the hub

○ Divide tasks between team members (parsing, output, algorithm, …)

● The perfect solution is not necessary (does not exist?)

● You must optimize○ Find a way to maximize/minimize a goal function

● Several techniques○ Dynamic Programming with approximations

○ Greedy Algorithms (choose the local optimal choice)

Dynamic Programming - a gentle introduction -

1

12

3 5

813

2134

Problem:

Compute Fibonacci forN OVER 9000!

static int fibonacci_1(int n)

1. static int fibonacci_1(int n) {

2. System.out.println("computing " + n);

3. if (n

main

1. public static void main(String[] args) {

2. Scanner sc = new Scanner(System.in);

3. int n = sc.nextInt();

4. System.out.println(fibonacci_1(n));

5. }

Ok, let’s try with 6

computing 6computing 5computing 4computing 3computing 2computing 1computing 2computing 3computing 2computing 1computing 4computing 3computing 2computing 1computing 28

6

Successione di Fibonacci - Stack delle chiamate

6 works, let’s try 100!

computing 100computing 99computing 98………computing 99……computing 98 ………

5 minutes later

Not yet finished

100

Successione di Fibonacci - Dynamic Programming

static int fibonacci_2(int n)

1. static Map cache_2 = new HashMap();

2.

3. static Integer fibonacci_2(int n) {

4. if (cache_2.containsKey(n))

5. return cache_2.get(n);

6.

7. System.out.println("computing " + n);

8. int result;

9.

10. if (n

What about 100 now?

computing 100computing 99computing 98computing 97computing 96computing 95…computing 5computing 4computing 3computing 2computing 1-980107325

100

static BigInteger fibonacci_3(int n)

1. static Map cache_3 = new HashMap();

2.

3. static BigInteger fibonacci_3(int n) {

4. if (cache_3.containsKey(n))

5. return cache_3.get(n);

6.

7. System.out.println("computing " + n);

8. BigInteger result;

9.

10. if (n

Let’s make fibonacci great again!

computing 100computing 99computing 98computing 97computing 96computing 95…computing 5computing 4computing 3computing 2computing 1354224848179261915075

100

GO BIG!

computing 1000computing 999computing 998computing 997computing 996computing 995…computing 5computing 4computing 3computing 2computing 143466557686937456435688527675040625802564660517371780402481729089536555417949051890403879840079255169295922593080322634775209689623239873322471161642996440906533187938298969649928516003704476137795166849228875

1000

GO BIIIIIIIIIIIIIIIIIIG!

computing 10000computing 9999computing 9998computing 9997computing 9996computing 9995…computing 4471

Exception in thread "main" java.lang.StackOverflowError … at com.company.Main.fibonacci_3

10000

Stack Overflow? Let’s increase the Stack Size!

just leave a bit of memory for the system,

But don’t fear to go to gigabytes!

GO BIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIGGG!!111

computing 10000computing 9999computing 9998computing 9997computing 9996computing 9995…computing 5computing 4computing 3computing 2computing 1

10000

GO BIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIGGG!!111

33644764876431783266621612005107543310302148460680063906564769974680081442166662368155595513633734025582065332680836159373734790483865268263040892463056431887354544369559827491606602099884183933864652731300088830269235673613135117579297437854413752130520504347701602264758318906527890855154366159582987279682987510631200575428783453215515103870818298969791613127856265033195487140214287532698187962046936097879900350962302291026368131493195275630227837628441540360584402572114334961180023091208287046088923962328835461505776583271252546093591128203925285393434620904245248929403901706233888991085841065183173360437470737908552631764325733993712871937587746897479926305837065742830161637408969178426378624212835258112820516370298089332099905707920064367426202389783111470054074998459250360633560933883831923386783056136435351892133279732908133732642652633989763922723407882928177953580570993691049175470808931841056146322338217465637321248226383092103297701648054726243842374862411453093812206564914032751086643394517512161526545361333111314042436854805106765843493523836959653428071768775328348234345557366719731392746273629108210679280784718035329131176778924659089938635459327894523777674406192240337638674004021330343297496902028328145933418826817683893072003634795623117103101291953169794607632737589253530772552375943788434504067715555779056450443016640119462580972216729758615026968443146952034614932291105970676243268515992834709891284706740862008587135016260312071903172086094081298321581077282076353186624611278245537208532365305775956430072517744315051539600905168603220349163222640885248852433158051534849622434848299380905070483482449327453732624567755879089187190803662058009594743150052402532709746995318770724376825907419939632265984147498193609285223945039707165443156421328157688908058783183404917434556270520223564846495196112460268313970975069382648706613264507665074611512677522748621598642530711298441182622661057163515069260029861704945425047491378115154139941550671256271197133252763631939606902895650288268608362241082050562430701794976171121233066073310059947366875

static BigInteger fibonacci_4(int n)

1. static Map cache_4 = new HashMap();

2.

3. static BigInteger fibonacci_4(int n) {

4.

5. Function fn = new Function() {

6. @Override

7. public BigInteger apply(Integer x) {

8. if (x

aaaand it’s done !

of course one should not use recursion for Fibonacci, but a loop… anyway…

1. static BigInteger fibonacci_5(int n) {

2. BigInteger a = BigInteger.ONE, b = BigInteger.ONE, next;

3. for (int i = 2; i < n; i++) { next = a.add(b); a = b; b = next; }

4. return b;

5. }

What about libraries?

● The first rule of Java Club is: use maven

● The second rule of Java Club is: USE MAVEN

Useful Java Libraries:

● Fastutil https://github.com/vigna/fastutil● ND4J http://nd4j.org/● Guava https://github.com/google/guava

https://github.com/vigna/fastutilhttp://nd4j.org/https://github.com/google/guava

How do I learn?

https://hackerrank.com

https://hackerrank.comhttps://hackerrank.com

seclab.unibg.it

http://seclab.unibg.it/http://seclab.unibg.it/

one more thing ...

https://hashcodejudge.withgoogle.com

https://hashcodejudge.withgoogle.comhttps://hashcodejudge.withgoogle.com

Unibg Seclab - Practice problem internal competition

The team that submits the highest scores for the practice problem gets free pizza during the competition.

=>

seclab.unibg.it

http://seclab.unibg.it/http://seclab.unibg.it/