By David/Denon

First of All…

Welcome guys

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Agenda

• Contest Format• Taste the challenges• Input /Output• STL introduction• Admin

THE CONTESTACM ICPC

The Contest

ACM International Collegiate Programming Contest (abbreviated as ACM-ICPC or just ICPC) is an annual multi-tiered computer programming competition among the universities of the world. The contest is sponsored by IBM. Headquartered at Baylor University, with autonomous regions on six continents, the ICPC is directed by Baylor Professor William B. Poucher, Executive Director, and operates under the auspices of the Association for Computing Machinery (ACM).(From wikipedia)

The Contest● World Finals is held every year.● To earn the free ticket to this glory battlefield, we need to

excel in the Regional competitions.– To be held in late October and November.

The Contest• Team of three• One Computer per

team• 8 – 13 problems• X hours

• Solve the most problem in the shortest time wins the competition

The Contest

while ( ! isTimeout() ) {read();think();program();//debug();submit();

}

One Balloon as a gift for solving a problem.Each problem is associated with a color.

HOW?Solve Problem

Let’s try

10080 - Gopher II563 - Crimewave

This is a interesting problem but solving it requires some knowledge (i.e. max flow) that you may or may not have acquired at the moment. However, in order for you to see what is a typical ICPC problem looks like, just pretend that you know how to solve the problem on the next slide.

Let’s Try

So Si

4 1

2 3 2

3

4

So = SourceSi = Sink

Let’s Try

So Si

0/4 0/1

1/2 1/3 1/2

0/3

0/4

So = SourceSi = Sink

Is this a possible setting?Does it violate any constraints?

Let’s Try

So Si

0/4 0/1

1/2 1/3 1/2

0/3

0/4

So = SourceSi = Sink

What is the total flow from the Source to the Sink?

Let’s Try

So Si

3/4 3/1

4/2 0/3 1/2

0/3

0/4

So = SourceSi = Sink

How about this one?Is it a valid configuration? (2)

Let’s Try

So Si

1/4 1/1

2/2 1/3 1/2

0/3

0/4

So = SourceSi = Sink

This is another valid configuration.

Max-Flow Problem

Max-Flow Problem• When given a graph, what is the max. valid

flow on it?

Max-Flow Problem

So Si

1/4 1/1

1/2 0/3 2/2

2/3

0/4

So = SourceSi = Sink

Max. Flow = 3

Max-Flow Problem

Now just assume that your teammate know how to program the function() to solve the max-flow problem.

Input: (single source, single sink, non-negative constraints)

So Si

4 1

2 3 2

3

4

Max-Flow Problem

Input: (single source, single sink, non-negative constraints)

S Si

4 1

2 3 2

3

4

Output: (Max total flow, flow on each edge)

S Si

1/4 1/1

1/2 0/3 2/2

2/3

0/4

Max-Flow Problem

• What if there are multiple sources or/and sinks? How can we use the function() to help solving the problem? (Remember function() can only solve problems of single sources and single sources)

So Si

41

2 3 2

3

4

So Si

4

4 3

Max-Flow Problem

• Transformation

S S

4 1

2 3 2

4

S S

4

4 3

Max-Flow Problem

• Transformation

S Si

4 1

2 3 2

4

S Si

4

4 3

S’Si’

Max-Flow Problem

• Transformation

SS SiSi

4 1

2 3 2

4

SS SiSi

4

4 3

S’Si’

Max-Flow Problem

• Transformation

SS SiSi

4 1

2 3 2

4

SS SiSi

4

4 3

S’Si’

infiinfi

infi infi

Can you solve the following problem?10080 - Gopher II

Hints: Use the function() Hints: do a transformation

10080 - Gopher II

Let’s have 5 mins break. (It is not actually a break. It is expected that you read the problem during this given time)

http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=12&page=show_problem&problem=1021

10080 - Gopher II Hints: Use the function()

Hints: do a transformation

HoleHole

HoleHole

HoleHole

5s Away

Velocity of Gopher: 1 m/s

3m(3s)

8m(8s)8m(8s)

3m(3s)

8m(8s)

3m(3s)

3m(3s)

3m(3s)

3m(3s)

10080 - Gopher II Hints: Use the function()

Hints: do a transformation

HoleHole

HoleHole

HoleHole

5s Away

Velocity of Gopher: 1 m/s

3m(3s)

8m(8s)8m(8s)

3m(3s)

8m(8s)

3m(3s)

3m(3s)

3m(3s)

3m(3s)

The Constraint is 1 Gopher per hole

10080 - Gopher II Hints: Use the function() Hints: do a transformation Hints: one Gopher = 1 unit of flow

HoleHole

HoleHole

HoleHole

The Constraint is 1 Gopher per hole

10080 - Gopher IISolution:

HoleHole

HoleHole

HoleHole

Source Sink

1

1

1

1

1

1

Let’s Try

• Want More?

563 - Crimewave

10 mins break. (Again, it is not actually a break. It is expected that you read the problem, and try to find the solution with your neighbor. )

http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=7&page=show_problem&problem=504

563 - Crimewave

http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=7&page=show_problem&problem=504

Banks

Get-away routes

City MapIs it possible that no two get-away routes use the same crossing?

563 - Crimewave

Hint: Use the function()

A

D

E

B

C

Only one route can pass through each node…

But we can have constraints on edge only…

563 - Crimewave

Hint: Use the function()

A

D

E

B

C

Only one route can pass through each node…

Banks as Sources.City Bounds as Sink.

563 - Crimewave

Hint: Use the function()

A

D

E

B

C

Is this okay?

1

1

11

1

1 1

1

563 - Crimewave

Hint: Use the function()

A1

D

E

B

C

Is this okay?

A21

563 - Crimewave

Hint: Use the function()

A1

D

E

B

C

Is this okay?

A21

A

Let’s Try

Don’t worry. It is normal that you cannot get the solution right away. These problems are really difficult. That’s why we have to meet every week.

Review

What processes were used when we solved the problem(s)?– Read and understand the problem– Transform the problem– Apply an “known” algorithm– Get back the solution

Review

What processes were used when we solved the problem(s)?– Read and understand the problem– Transform the problem– Apply an “known” algorithm– Get back the solution– Code the problem– Way to represent the graph in the program (Data-

Structure)

Review

What processes were used when we solved the problem(s)?– Read and understand the problem– Transform the problem– Apply an “known” algorithm– Get back the solution– Code the problem– Way to represent the graph in the program (Data-

Structure)

INPUT /OUTPUTLet’s get the hands dirty

Let’s get the hands dirty

• 483 - Word Scramble– http://uva.onlinejudge.org/index.php?

option=com_onlinejudge&Itemid=8&category=6&page=show_problem&problem=424

This is a warm-up problem. You have to code this time.

Start your notepad/ DEV C++/ vi/ gedit …

483 - Word Scramble

• Common way to read lines string line;while (getline(cin, line)){ // .. to process a line}

483 - Word Scramble

● For each line, we need to get the word (Token).

istringstream ins(line);string word;while (ins >> word){ // reverse word}

483 - Word Scramble

● The last part is to reverse a word.● You may instantly thinking about a loop to

do this.

● Let’s try the STL reverse() function.

for (size_t i = 0; i < words.size(); ++i) cout << words[words.size() - i - 1];

reverse(word.begin(), word.end());cout << word;

reverse(…, …)

• What does the reverse(…, …) actually do?1.Go to http://cplusplus.com/2.Search reverse.

reverse(…, …)

• void reverse ( BidirectionalIterator first, BidirectionalIterator last);

H E L L O

First( String.begin() )

Last( String.end() )

reverse(…, …)H E L L O

First( String.begin() )

Last( String.end() )

Parametersfirst, last Bidirectional iterators to the initial and final positions of the

sequence to be reversed. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.

STL

Standard Template Library• Provides common library functions like

reverse(…, …)• More Example,

1. Go to http://cplusplus.com/2. See Reference STL Algorithms

STL

Standard Template Library• Provides common container classes• Hey, but, but what is container class?

STL

Standard Template Library• Array is an example of container, but it has

limited functionality such as– The size is fixed– The index must be non-negative integer

STL

Standard Template Library• When I was a child, I always dreamt to have a

non-fixed size array, now the dream comes true. It is called vector

Vector#include <iostream> #include <vector> using namespace std;

int main () { vector<int> myvector (10); // 10 zero-initialized elements

// assign some values: myvector[9] = 5;cout << myvector[9] << endl;

myvector.push_back (15);myvector.push_back (16);

cout << myvector[10] << “, “ << myvector[11] << endl; // 15, 16}

STL

Standard Template Library• When I was a child, I also always dreamt to

have an array that can use “string”, “char”, … as its index.– table[“denon”] = “smart”;– table[“david”] = “stupid”;

• Now the dream comes true again, it is called map

Map#include <iostream> #include <map> using namespace std;

int main () { map<char,int> table; first['a']=10; first['b']=30; first['c']=50;

cout << table[‘a’] << “, “ << table[‘c’] << endl; //10, 50

return 0; }

STL

• More containers and algorithms will be covered in the following trainings

• For more information,– http://cplusplus.com/– http://www.sgi.com/tech/stl

Exercises

• 642 - Word Amalgamation• 401 - Palindromes• 10107 - What is the Median?

ADMINHere is the boring part…

Admin

Website: http://www.cs.hku.hk/~provinci

Mailing list: provinci@cs.hku.hk

Coach: Chan Ho Leung

Email: hlchan@cs.hku.hk

Admin

• We’ll have a Mini Contest 1. test at Mar 11 (Thur) 18:30-21:00.

● To prepare for the Mini Contest 1, you can join our training sessions every Thursday in HW-311.– These training sessions are optional and you can

join the contest directly.

Admin

• Tentative schedule

http://i.cs.hku.hk/~provinci/training.html

PRACTICE MAKE PERFECTFinally…

Practice make perfect

● An archive of problems can be found in the site– http://acm.uva.es/p

● It also has an on-line judge to test your submitted program.– Simply register and apply for an account.

● This archive site will be a major source of training questions.

Practice make perfect

● Useful resources…

http://ctldenon.net/resources.php

Warm-up problems:• 642 - Word Amalgamation• 401 - Palindromes• 10107 - What is the Median?

That’s All

Thank you