GB Programming Challenges

Transcription

1 GB Programming Challenges Week 1 - Ad-hoc problems Claus Aranha caranha@cs.tsukuba.ac.jp College of Information Science April 18, 2014

2 Some Notes Before the Class Don t forget to send me your username Report submission will happen on the programming-challenges webpage, so please make sure to send me your username. Course Webpage All the lecture notes for the course, as well as probem listings and important notices, will be placed on the course webpage:

3 How was your weekend? If a few of you tried to play with the submission site this weekend, let us know about it! Did anyone solve a problem during this weekend?

4 How was your weekend? If a few of you tried to play with the submission site this weekend, let us know about it! Did anyone solve a problem during this weekend? Did anyone try to solve a problem?

5 How was your weekend? If a few of you tried to play with the submission site this weekend, let us know about it! Did anyone solve a problem during this weekend? Did anyone try to solve a problem? Did anyone read some problems?

6 How was your weekend? If a few of you tried to play with the submission site this weekend, let us know about it! Did anyone solve a problem during this weekend? Did anyone try to solve a problem? Did anyone read some problems? Did anyone had problems accessing the submission page?

7 Today s Class Advice on Solving Programming Challenges Ad-hoc types of problems Discussing this week s problems

8 Today s Class Advice on Solving Programming Challenges What should you pay attention to when solving P.C.? What are the common mistakes? Special consideration for the JAVA programming language Ad-hoc types of problems Discussing this week s problems

9 Today s Class Advice on Solving Programming Challenges Ad-hoc types of problems Ad-hoc problems don t need special kind of algorithms, but can be solved by following the instructions carefully. We will dicuss some examples and talk about how to solve them Discussing this week s problems

10 Today s Class Advice on Solving Programming Challenges Ad-hoc types of problems Discussing this week s problems Let s talk about this week s problems. Good time to ask questions!

11 Part 1: Reading the specification Read the entire problem description Be careful with assumptions Look out for Worst case scenarios First write down the algorithm Before anything, read the entire problem description completely! Sometimes hints and limits are hidden in the fluff text. Mark keywords. Write the problem outline as you read. The most important thing is to try to identify the central concept. Most problems are focused on one idea (graph, arithmetic, geometry, etc) If you have difficulties with English, this is very important!

12 Part 1: Reading the specification Read the entire problem description Be careful with assumptions Look out for Worst case scenarios First write down the algorithm Graphs: Connected? Directed? Redundant Edges? Negative weights?

13 Part 1: Reading the specification Read the entire problem description Be careful with assumptions Look out for Worst case scenarios First write down the algorithm Geometry: Overlapping? Concave? Negative coordinates? Collinear?

14 Part 1: Reading the specification Read the entire problem description Be careful with assumptions Look out for Worst case scenarios First write down the algorithm Worst Cases Maximum number of entries; Zero entries; Wrong sort order; Unsolvable Entries; Empty sets; Duplicate items; Big Numbers (> Long Long);

15 Part 1: Reading the specification Read the entire problem description Be careful with assumptions Look out for Worst case scenarios First write down the algorithm Implementation distracts from problemsolving!

16 Part 2: Input/Output First thing: Get the input/output right So you can write/test everything else Release Often philosophy: Start with multiple small changes, check that each of them work Check Input conditions: Is the input terminated by EOF, or special data values? Do you need to deal with newlines, or every case is in one line? Do you need to deal with strings, or just integers?

17 Part 2: Input/Output C++ #include<iostream> int main() { long long a,b,c; while (std::cin>>a>>b) { if (b>a) c = b - a; else c = a - b; std::cout << c << endl; } }

18 Part 2: Input/Output Java - classes The entire submission must be in a single file; The judge calls a Main class for execution; Do not declare classes public ; Use as many classes as you need (in a single file) Java - libraries java::io is restricted; functions related to networking and threads are also restrictred; java::util and java::math can (should!) be used at will;

19 Part 2: Input/Output Program Skeleton for Java import java.io.*; import java.util.*; class Main implements Runnable{ static String ReadLn(int maxlength){ // utility function to read from stdin, // Provided by Programming-challenges, edit for style only byte line[] = new byte [maxlength]; int length = 0; int input = -1; try{ while (length < maxlength){//read untill maxlength input = System.in.read(); if ((input < 0) (input == \n )) break; //or untill end of line ninput line [length++] += input; } } if ((input < 0) && (length == 0)) return null; return new String(line, 0, length); }catch (IOException e){ return null; } } //(... continues next page...) // eof

20 Part 2: Input/Output Program Skeleton for Java class Main implements Runnable{ //(... continuation...) public static void main(string args[]) // entry point from OS { Main mywork = new Main(); // Construct the bootloader mywork.run(); // execute } } public void run() { new mystuff().run(); } class mystuff implements Runnable{ public void run(){ // Your program here } } // You can insert more classes here if you want.

21 Part 3: Programming Each language has its own gimmicks: C,C++: Be careful of runtime errors. Most of runtime errors in programming contests happens with this language. Usually because of pointer schenanigans Love the SDL. Java: Java was not really designed for this kind of use - compiling erros Ahoy! Get used to the idioms necessary for Programming Contest Java;

22 Part 4: Debugging I got a Wrong Answer, but I m sure my program is right! No, you are failing the secret tests Write your own hard tests! Did you try tests with the limit conditions? (Max values, Min Values) Did you try tests with random data? Check all the assumptions of your code - are they supported by the problem description? In the last (of the last!) cases, there might be a judge error. Contact the professor. DON T PANIC!

23 What are ad-hoc problems There problems don t involve complex algorithms. They need a kind of follow the orders program to be solved. This does not mean that they are always easy to solve: Many rules and special input/output may be involved. Some times they are called Simulation problems. Common Adhoc problem pattern A system is described. Start condition and changes are described. Given a list of changes, calculate the final condition.

24 Example 1: Zero or One game Let s see the PDF; Extremely trivial problem, we should try to solve it now in class. There are two winning conditions - a 1 and two zeroes, and a zero and two ones. Try to solve it in 5 minutes!

25 Example 1: Zero or One game Let s see the PDF; Extremely trivial problem, we should try to solve it now in class. There are two winning conditions - a 1 and two zeroes, and a zero and two ones. Try to solve it in 5 minutes! What if we increased the number of players? What would change in the algorithm?

26 Example 2: The Luggage For this problem, we must observe each input, and print a solution for each of them. Pay Attention! The total number of inputs is given as the first element in the input. Pay Attention! After the problem is ended, we still need to print the final grand total.

27 Hints for solving ad-hoc problems Keep your program simple - Ad-hoc problems tend to have many moving parts, and it can become a challenge to keep track of everything your program is doing. Favor arrays over pointers. Learn string manipulation functions.

28 The 3n+1 Problem

29 Minesweeper

30 The Trip

31 Interpreter