Transcript Programming Interest Group - Department of Computer

Programming Interest Group
Tutorial One
Introduction:
Get Familiar with Your Weapon
1
What is ACM?
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ACM: Association for Computing Machinery
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http://www.acm.org/
the world’s largest educational and scientific
computing society
ACM ICPC
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ACM International Collegiate Programming
Contest
http://en.wikipedia.org/wiki/ACM_International_Co
llegiate_Programming_Contest
2
ACM ICPC
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ICPC is a two-tiered competition among
teams of students representing institutions of
higher education.
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Teams compete in Regional Contests, from which
top scoring teams advance to the ACM-ICPC
World Finals.
Each team has three students, sharing one
computer, given a number of programming
problems
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Coordination and teamwork are essential
3
Online Judge (OJ)
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When you finish a problem, submit the source code to the online
judge
 You can use C, C++, Java
 The online judge will compile your source code, and check with
unknown number of secret test cases
Feedback from the judge
 Accepted (AC) – congratulations!
 Presentation Error (PE) – Your program outputs are correct, but
are not presented in the specified format. Check for spaces,
left/right justification, line feeds, etc.
 Wrong Answer (WA) – Your program returned an incorrect
answer to one or more of the judge’s secret test cases
 Compile Error (CE) – The judge’s compiler cannot compile your
source code
4
Online Judge (cont.)
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Runtime Error (RE) – Your program failed
during execution due to a segmentation fault,
floating point exception, or others.
Time Limit Exceeded (TL) – Your program
took too much time on at least one of the test
cases. Try to improve the efficiency of your
solution!
Memory Limit Exceeded (ML) – Your
program tried to use more memory than the
judge’s settings.
5
Available OJs
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There are many famous online judges
 Valladolid OJ (http://acm.uva.es/p)
 Ural OJ (http://acm.timus.ru)
 Saratov OJ (http://acm.sgu.ru)
 ZJU OJ (http://acm.zju.edu.cn)
 ZJUT OJ (http://acm.zjut.edu.cn)
 Official ACM Live Archive (http://cii-judge.baylor.edu/)
 Peking University Online Judge
(http://acm.pku.edu.cn/JudgeOnline/)
 Programming Challenges (http://www.programmingchallenges.com)
6
What do we use?
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Programming Challenges
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Steven S. Skiena and Miguel Revilla,
Programming Challenges, the Programming
Contest Training Manual, Springer, 2003.
http://www.programmingchallenges.com/pg.php?page=index
Our main training reference, available from library
7
Suggested Books
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Art of Programming Contest (free online)
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http://onlinejudge.uva.es/p/Art_of_Programming_Contest_SE_for_uva.pdf
Thomas H. Cormen, Charles E. Leiserson, Ronald L.
Rivest, and Clifford Stein, Introduction to Algorithms, 2nd
Edition, The MIT Press, 2001.
Robert Sedgewick, Bundle of Algorithms in Java, Third
Edition (Parts 1-5), 3rd Edition, Addison-Wesley
Professional, 2003. (There is also a C++ version).
Donald E. Knuth,The Art of Computer Programming,
Volume 1, 2, 3.

Bill Gates said that in order to be "a really good programmer"
one must read its first volume and solve all of its problems before
sending his company a resume.
8
Programming Languages
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Machine Language
Or called Machine code, understandable directly
by CPU
 Different CPU has different Instruction Set
 A program is just a sequence of instructions that
are executed by a CPU.
 E.g.: adding registers 1 and 2 and placing the
result in register 6 is encoded as:
000000 00001 00010 00110 00000 100000
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9
Machine Language
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Shall we understand the so cold binary code?
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CPU designer
Writing an assembler
Reverse-engineering
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http://www.reverse-engineering.net/
A book:
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Hacker Disassembling Uncovered by Kris Kaspersky
10
Assembly Language
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The same as machine language. But the
command numbers are replaced by letter
sequences, understandable by CS students.
Assembler: transform the assembly language
into machine code
E.g.:
movl $1, %eax
movl $0, %ebx
int $0x80
11
Assembly Language
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Shall we learn assembly language today?
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Write an Operating System
Write a compiler
Optimize the performance of a piece of program
MMX, SSE, SSE2, etc.
Help us to understand how computer works!
A free book: Programming from the Ground
Up, by Jonathan Bartlett
12
High-level Language
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Allow us to describe the program in a more
natural language.
A comparison of different programming
languages:
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http://en.wikipedia.org/wiki/Comparison_of_progra
mming_languages
Compilation vs. Interpretation
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Interpreters are generally slower to run, but more
flexible than compilers.
13
C
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Quoted from wikipedia:
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The C programming language (often, just "C") is a
general-purpose, procedural, imperative computer
programming language developed in the early 1970s by
Dennis Ritchie for use on the Unix operating system.
K&R C
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In 1978, Dennis Ritchie and Brian Kernighan published the
first edition of The C Programming Language. This book,
known to C programmers as "K&R," served for many years
as an informal specification of the language. The version of
C that it describes is commonly referred to as "K&R C."
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C
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ANSI C and ISO C
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The first standard for C was published by ANSI.
After a long and arduous process, the standard was
completed in 1989 and ratified as ANSI X3.159-1989
"Programming Language C." This version of the language
is often referred to as ANSI C, or sometimes C89.
In 1990, the ANSI C standard (with a few minor
modifications) was adopted by the International
Organization for Standardization (ISO) as ISO/IEC
9899:1990. This version is sometimes called C90.
Therefore, the terms "C89" and "C90" refer to essentially
the same language.
ANSI C is now supported by almost all the widely used
compilers.
15
C
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C99
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After the ANSI standardization process, the C language
specification remained relatively static for some time,
whereas C++ continued to evolve.
A new C standard is finally published as ISO 9899:1999 in
1999. This standard is commonly referred to as "C99." It
was adopted as an ANSI standard in March 2000.
Some new features:
 Inline functions
 Variables can be declared anywhere
 Variable-length arrays
 Support for one-line comments beginning with //
16
C Books
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C Programming Language, 2nd Edition, by
Brian W. Kernighan and Dennis M. Ritchie.
Expert C Programming: deep C secrets, by
Peter van der Linden.
The Standard C Library, by P. J. Plauger.
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C++
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Stroustrup began work on C with Classes in 1979.
In 1983, the name of the language was changed from C with
Classes to C++.
In 1985, the first edition of The C++ Programming Language was
released.
As the C++ language evolved, a standard library also evolved
with it, which finally leaded to Standard Template Library (STL).
After years of work, a joint ANSI-ISO committee standardized
C++ in 1998 (ISO/IEC 14882:1998).
 The 1998 C++ standard consists of two parts: the core language
and the C++ standard library; the latter includes most of the
Standard Template Library and a slightly modified version of the
C standard library.
A newer version of C++ standard was published in 2003.
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C++
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C++ can be thought of as comprising three
parts
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The low-level language, largely from C
Object-Oriented programming: to define our own
data types and to organize large-scale programs
and systems
Standard library: to provide a set of useful data
structures and algorithms
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C++
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Templates and Generic programming
Generic programming involves writing code in a way
that is independent of any particular type.
Templates are the foundation of generic
programming.
int compare(const string &v1, const string &v2)
int compare(const double &v1, const double &v2)
{
{
}
if (v1 < v2) return -1;
if (v1 < v2) return -1;
if(v2 < v1) return 1;
if(v2 < v1) return 1;
return 0;
return 0;
}
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C++
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The following is a template version of
compare:
Template <typename T>
int compare(const T &v1, const T &v2)
{
if (v1 < v2) return -1;
if(v2 < v1) return 1;
return 0;
}
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C++ Books
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C++ Primer
C++ Primer Plus
Thinking in C++
The C++ Programming Language, by Bjarne
Stroustrup
The C++ Standard Library: A Tutorial and
Reference , by Nicolai M. Josuttis
22
Java
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Java was started as a project called "Oak" by
James Gosling in June 1991.
The first public implementation was Java 1.0
in 1995. It made the promise of "Write Once,
Run Anywhere" (WORA), with free runtimes
on popular platforms.
Java remains a proprietary de facto standard
that is controlled through the Java
Community Process
(http://www.jcp.org/en/home/index).
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Java
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Object-Oriented programming
Platform independence
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It means that programs written in the Java language must
run similarly on diverse hardware. One should be able to
write a program once and run it anywhere.
Java language code  Java bytecode
Jave bytecode is then run on a virtual machine (VM), a
program written in native code on the host hardware that
interprets and executes generic Java bytecode.
To improve the performance, Just-in-time (JIT)
compilation has been introduced, by translating bytecode
into native machine code at runtime.
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Java Libraries
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Core libraries:
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Integration libraries:
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The Java Database Connectivity (JDBC) API for database access
Java Naming and Directory Interface (JNDI) for lookup and discovery
RMI and CORBA for distributed application development
User interface libraries:
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Data structures
XML parsing libraries
Security
Internationalization and localization
AWT (Abstract Windowing Toolkit )
Swing
APIs for audio capture, processing, and playback
Lots of extensions:
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Java EE (J2EE), Java ME (J2ME)
JMF (Java Media Framework)
JAI (Java Advanced Imaging)
Java 3D, JOGL (Java OpenGL)
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Others
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Python
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Rumor: advocated by Google
"Python has been an important part of Google since the
beginning, and remains so as the system grows and
evolved. Today dozens of Google engineers use Python,
and we're looking for more people with skills in this
language"
-- Peter Norvig, Director of Search Quality at Google
“I am the author of the Python programming language. …In
December 2005 I joined Google in Mountain View, CA.”
-- Guido van Rossum, Creator of Python
C#
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Developed by Microsoft (to combat Java)
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Standard Input/Output
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In ACM contest, each program must read the
test data from the standard input and print the
results to the standard output
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For C language, use scanf() and printf()
For C++, use cin and cout
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scanf() and printf() are also supported
For Java, refer to http://www.programmingchallenges.com/pg.php?page=javainfo
Programs are not allowed to open files or to
execute certain system calls
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Not convenient for debugging?
#include <stdio.h>
int main ()
{
freopen(“FILE_NAME_FOR_INPUT”,”r”,stdin);
freopen(“FILE_NAME_FOR OUTPUT”,”w”,stdout);
Rest of the codes…
return 0;
}
While sending your code to online judges,
remember to remove the two lines with freopen.
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Simple Coding
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Avoid the usage of the ++ or -- operators inside
expressions or function calls
Avoid expressions of the form *p++
Avoid pointer arithmetic. Instead of (p+5) use
p[5].
Never code like : return (x*y)+Func(t)/(1-s);
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but like :
 temp = func(t);
 RetVal = (x*y) + temp/(1-s);
 return RetVal;
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Naming
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Don’t use small and similar names for your
variables. Use descriptive names.
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Don’t use names like {i,j,k} for loop control
variables. Use {I,K,M}.
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It is very easy to mistake a j for an i when you
read code or “copy, paste & change” code,
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Internal force
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Data Structures
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Commonly seen in almost every programming
projects
A treasure from tens of years of experiences
Summarized by computer scientists as textbooks
A core subject for every computer
science/computer engineering department
31
Internal force
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Algorithms
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To solve real problems efficiently
Categories:
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Sorting
Searching
Graph algorithms
Scientific computing: matrix, number-theoretic,
computational geometry, etc.
…
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Internal force
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Mathematics
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Everything finally goes back to mathematics!
Number theory
Geometry
Combinatorics
Graph theory
…
33
A Good Team
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three factors crucial for being a good
programming team
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Knowledge of standard algorithms and the
ability to find an appropriate algorithm for
every problem in the set;
Ability to code an algorithm into a working
program;
Having a strategy of cooperation with your
teammates
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Quickly identify problem types
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In programming contests, you will be dealing with a
set of problems, not only one problem.
Problem types
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Mathematics (number theory, big integer, etc)
Sorting
Searching
Simulation
String processing
Dynamic programming (DP)
Graph
Computational geometry
Ad Hoc (i.e., no standard categories)
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Analyze your algorithm
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Proof of algorithm correctness
Time/Space complexity analysis for non recursive
algorithms
For recursive algorithms, the knowledge of
computing recurrence relations and analyze them:
iterative method, substitution method, recursion tree
method and finally, Master Theorem
Given the maximum input bound (usually given in
problem description), can my algorithm, with the
complexity that I can compute, pass the time limit
given in the programming contest?
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Some rules of thumb
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Biggest built in data structure "long long" is
2^63-1: 9*10^18 (up to 18 digits)
If you have k nested loops running about n
iterations each, the program has O(nk)
complexity
The best times for sorting n elements are
O(nlog n)
DP algorithms which involves filling in a matrix
usually in O(n^3)
In contest, most of the time O(n log n) algorithms
will be sufficient
37
Testing your code
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You won’t get any credit by partially solving the
problem.
You need to be able to design good test cases
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Sample input-output given in problem description is by
default too trivial to measure your code's correctness
Before submission, you may want to design
some tricky test cases first, test them in your
own machine, and ensure your code is able to
solve them correctly
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Guidelines in designing good
test cases
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Must include sample input, the most trivial one
Must include boundary cases, what is the maximum n,x,y, or other input
variables, try varying their values to test for out of bound errors
For multiple input test case, try using two identical test case consecutively.
Both must output the same result. This is to check whether you forgot to
initialize some variables
Increase the size of input. Sometimes your program works for small input
size, but behave wrongly when input size increases.
Tricky test cases, analyze the problem description and identify parts that are
tricky, test them to your code.
Don‘t assume input will always nicely formatted if the problem description
didn’t say so. Try inserting white spaces (space, tabs) in your input, check
whether your code is able to read in the values correctly
Finally, do random test cases, try random input and check your code's
correctness
39
Producing Winning Solution
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Write down a game plan for what you're going to do in a
contest round
Read through all the problems first, don't directly attempt
one problem since you may missed easier problem.
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Order the problems: shortest job first, in terms of your effort
Sketch the algorithms, complexity, the numbers, data structures,
tricky details.
Brainstorm other possible algorithms
Do the Math! (space & time complexity)
Code it of course, as fast as possible, and it must be correct
Try to break the algorithm - use special (degenerate?) test cases.
40
Coding a problem
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Only coding after you finalize your algorithm.
Create test data for tricky cases.
Code the input routine and test it (write extra output routines to show
data).
Code the output routine and test it.
Write data structures needed.
Stepwise refinement: write comments outlining the program logic.
Fill in code and debug one section at a time.
Get it working & verify correctness (use trivial test cases).
Try to break the code - use special cases for code correctness.
41
Tips & tricks for contests
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Brute force when you can, Brute force algorithm tends to be the
easiest to implement.
KISS: Simple is smart! (Keep It Simple, Stupid !!! / Keep It Short &
Simple).
Hint: focus on limits (specified in problem statement).
Waste memory when it makes your life easier (trade memory space
for speed).
Don't delete your extra debugging output, comment it out.
Optimize progressively, and only as much as needed.
Keep all working versions!
42
Tips & tricks for contests
(Cont.)
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Code to debug:
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a. white space is good,
b. use meaningful variable names,
c. don't reuse variables, (we are not doing software engineering here)
d. stepwise refinement,
e. Comment before code.
Avoid pointers if you can.
Avoid dynamic memory like the plague: statically allocate
everything.
Try not to use floating point; if you have to, put tolerances in
everywhere (never test equality)
43
Some Google interview
questions
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Given a function which produces a random integer in the range 1
to 5, write a function which produces a random integer in the
range 1 to 7.
Find the intersection of 2 sorted integer arrays. What if one of
them is huge? What if one of them is so huge, it can't fit in
memory. How do you minimize the number of disk seeks?
Given a string A, how do you find all the repeated substrings with
minimum size of 2?
Given N computers networked together, with each computer
storing N integers, describe a procedure for finding the median of
all of the numbers. Assume that a computer can only hold O(N)
integers (i.e. no computer can store all N^2 integers). Also
assume that there exists a computer on the network without
integers, that we can use to interface with the computers storing
the integers .
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Example: A+B Problem
http://acm.zju.edu.cn/onlinejudge/showProblem.do?problem
Code=1001
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Time Limit:1 second Memory Limit:32768 KB
Description: Calculate a + b
Input:
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Output:
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The input will consist of a series of pairs of integers a and
b, separated by a space, one pair of integers per line.
For each pair of input integers a and b you should output
the sum of a and b in one line, and with one line of output
for each line in input.
Sample Input:
15
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Sample Output:
6
45
Example Solution
/* C code */
#include “stdio.h”
/* Java code */
import java.util.Scanner;
int main()
{
int a, b;
while (scanf(“%d %d”, &a, &b)
!= EOF) {
printf(“%d\n”, a+b);
}
return 0;
}
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
while (in.hasNextInt()) {
int a = in.nextInt();
int b = in.nextInt();
System.out.println(a + b);
}
}
}
46
Fundamental Problems
http://acm.zjut.edu.cn
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1167
1166
1174
1175
1176
1177
1178
1179
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1181
1185
1190
1191
1187
1204
1208
1205
1044
47
Fundamental Problems
http://acm.uva.es
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http://acm.uva.es/p/v1/100.html
http://acm.uva.es/p/v101/10189.html
http://acm.uva.es/p/v101/10137.html
http://acm.uva.es/p/v7/706.html
http://acm.uva.es/p/v102/10267.html
http://acm.uva.es/p/v100/10033.html
http://acm.uva.es/p/v101/10196.html
http://acm.uva.es/p/v101/10142.html
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3n+1 Problem
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The 3n+1 problem
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This is an outstanding unsolved problems in number
theory, called 3n+1 conjecture.
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http://acm.uva.es/p/v1/100.html
Start with an integer n;
If n is even, n=n/2; else n = 3n+1;
Repeat this process, terminating when n=1.
It’s conjectured that this algorithm will terminate at n=1 for
every integer n.
E.g.:
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22 11 34 17 52 26 13 40 20 10 5 16 8 4 2 1
The “cycle length” for 22 is 16.
49
3n+1 problem
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Write a function to calculate
the “cycle length” for integer i
int cycle_length(int i)
{
int length = 1;
while (i != 1) {
if( i % 2 == 0)
i = i>>1;
else
i = i<<1 + i + 1;
length++;
}
return length;
}

Write a function to calculate the
maximum cycle for integers
between and including i and j
void solve(int i, int j)
{
int t, m, n;
int length, maxlength;
if( i <= j) {
m = i;
n = j;
} else {
m = j;
n = i;
}
maxlength = 1;
for(t = m; t <= n; t++) {
length = cycle_length(t);
if (length > maxlength)
maxlength = length;
}
printf("%d %d %d\n", i, j, maxlength);
}
50
The trip
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http://acm.uva.es/p/v101/10137.html
A number of students are members of a club that travels annually
to exotic locations.
The group agrees in advance to share expenses equally, but it is
not practical to have them share every expense as it occurs. So
individuals in the group pay for particular things, like meals,
hotels, taxi rides, plane tickets, etc.
After the trip, each student's expenses are tallied and money is
exchanged so that the net cost to each is the same, to within one
cent. In the past, this money exchange has been tedious and
time consuming.
Your job is to compute, from a list of expenses, the minimum
amount of money that must change hands in order to equalize
(within a cent) all the students' costs.
51
The trip (Cont.)
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The Input
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Standard input will contain the information for several trips. There are no
more than 1000 students and no student spent more than $10,000.00. A
single line containing 0 follows the information for the last trip.
The Output
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For each trip, output a line stating the total amount of money, in dollars and
cents, that must be exchanged to equalize the students' costs.
Sample Input
Sample Input
3
10.00
20.00
30.00
0
Output: $10.00
4
15.00
15.01
3.00
3.01
0
52
Output: $11.99
The trip (Cont.)
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It’s important to analyze the problem first.
After you find the optimal way which can achieve the
“minimum amount of money that must change
hands”, writing the program will be very easy.
What’s the difficulty of this problem?

We need to equalize the costs as much as possible. That
means:
 if A pays the most, and A pays PA; if B pays the least, and
B pays PB,
 We need to have PB <= PA <= PB+1
53
The trip (Cont.)
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Assume there are n students. We can
calculate the total money.
If the total money is a multiple of n, then we
can equalize the costs. The problem is easy
to solve.
Example 1: 10.00, 20.00, 30.00
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Total = 60.00, average = 20.00
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The first student receives 10.00 from the third student
The second student doesn’t need to do anything.
The answer will be $10.00
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The trip (Cont.)
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If the total money is not a multiple of n
Let start from an example:
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1.10, 2.10, 3.10, 10.00
Total = 16.30 = 4x4.07 + 0.02
The final cost should be 4.07, 4.07, 4.08, 4.08
To minimize the amount of money change:
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
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1.10  4.07; 2.10  4.07; 3.10  4.08; 10.00 4.08
The answer will be $5.92
Question: do we need to sort the cost?
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