FAQ of OnlineJudge, Wuhan Univ.

Q: What does my program input from and output to?

A: Your program shall always input from stdin (Standard Input) and output to stdout (Standard Output). For example, you use scanf in C or cin in C++ to read from stdin, and printf in C or cout in C++ to write to stdout.

User programs are not allowed to open and read from/write to files. You will probably get a Runtime Error or Wrong Answer if you try to do so.

Input/Output of scanf/printf is faster than cin/cout, so if a problem has huge input data, use cin will probably get a Time Limit Exceeded.

Here is a sample solution to problem 1001 using G++:

#include <iostream>

using namespace std;

int main()
{
    int a, b;
    cin >> a >>  b;
    cout << a + b <<  endl;
    return 0;
}

Please note that the return type of main() must be int when you use G++/GCC, or else you may get Compile Error. Here is a sample solution to problem 1001 using GCC:

#include  <stdio.h>

int main()
{
    int a, b;
    scanf("%d %d", &a, &b);
    printf("%d\n", a + b);
    return 0;
}

Here is a sample solution to problem 1001 using PASCAL:

PROGRAM p1001(Input, Output); 

VAR
    a, b:Integer; 
BEGIN
    Readln(a, b); 
    Writeln(a + b); 
END.

Here is a sample solution to problem 1001 using JAVA:

The Java compiler is jdk 1.5 recently, below is a program for problem 1001

import java.io.*;
import java.util.*;
public class Main
{
    public static void main(String args[]) throws Exception
    {
        Scanner cin=new Scanner(System.in);
        int a = cin.nextInt(), b = cin.nextInt();
        System.out.println(a + b);
    }
}

Old program for jdk 1.4

import java.io.*;
import java.util.*;

public class Main
{
    public static void main (String args[]) throws Exception
    {
        BufferedReader stdin = 
            new BufferedReader(
                new InputStreamReader(System.in));

        String line = stdin.readLine();
        StringTokenizer st = new StringTokenizer(line);
        int a = Integer.parseInt(st.nextToken());
        int b = Integer.parseInt(st.nextToken());
        System.out.println(a + b);
    }
}

Q: How to use long long or __int64?

A: If you use long long in your GCC/G++ program, make sure you use %lld when read or write long long value. As __int64 is for MS-VC++, it is not supported in this System.

Q: How to copy my codes out of this Judge?

A: As some JudgeOnline System, the codes of all users' submissions are saved and others can see it. We allowed all users who passed a problem to see the codes of all submissions of this problem, and copy codes from the submissions status is not allowed.

Q: How to submit Java programs?

A: Java programs submitted must be in a single source code (not .class) file. They must also read from standard input and write to standard output, as in the other lang uages.

All programs must begin in a static method named main in a class named Main. However, you can implement and instance as many classes as needed.

Q: What are the meanings of the judge's replies?

A: Here is a list of the judge's replies and their meanings:
Running: The judge is so busy that it can't judge your submission or your submission is running at the moment. Usually you just need to wait several seconds and your submission will be judged.
Accepted (AC): Congratulations! Your program is correct!
Presentation Error (PE): Your output format is not exactly the same as the judge's output, although your answer to the problem is correct. Check your output for spaces, blank lines, etc. against the problem output specification.
Wrong Answer (WA): Expected output not reached for the input. The input and output data that we use to test the programs are not public (it is recommendable to get accustomed to a true contest dynamic ;-). For some problem Presentation Error is considented as Wrong Answer if the problem is just test your presentation.
Runtime Error (RE): Your program failed during the execution (illegal file access, stack overflow, pointer reference out of range, floating point exception, division by zero...).
   The following messages will not be shown to you in contest. Here we just provide some tips:
      SIGSEGV --- Segment Fault. The possible cases of your encountering this error are:
         1.buffer overflow --- usually caused by a pointer reference out of range.
         2.stack overflow --- please keep in mind that the default stack size is 8192K(Notice: In GCC a little overflow will not get a Runtime Error).
         3.illegal file access --- file operations are forbidden on our judge system.
      SIGFPE --- Divided by 0
      SIGBUS --- Hardware Error. //please contact us
      SIGABRT --- Programme aborted before it should be finished.
   man 7 signal under Linux for more information

   Free Pascal Runtime Error Numbers are refer to here http://community.freepascal.org:10000/docs-html/user/userch14.html for detailed runtime error informations. We list some frequently used error numbers here:
      200 Division by zero
      201 Range check error
      202 Stack overflow error
      203 Heap overflow error
      204 Invalid pointer operation
      205 floating point overflow
      206 floating point underflow
      207 invalid floating point operation
      216 General Protection fault
   Both Runtime Error(Pascal) and Runtime Error(JAVA) is allow you to click on the link at the Judge's reply to see the actual error message.

Time Limit Exceeded (TLE): Your program tried to run for too much time.
   Now each problem have two time limits - TOTAL TIME LIMIT and CASE TIME LIMIT. When our judge runs your program, it redirects your program's standard input to the input files, then you can just read from standard input. One problem may have several input files. When your program finishes one input file and produce correct output, the judge then restarts your program to deal with next input file. Each input file's format is exactly as the problem's input specification. The CASE TIME LIMIT is the maximum time your program is allowed to use for a single input file, and the TOTAL TIME LIMIT is the maximum time for your program to handle all input files. If your program exceed any of these two limits, you will receive Time Limit Exceeded.
   As most problems only have one or two input files (one or two corresponding output files of course), then its CASE TIME LIMIT is trivially set be the same as the TOTAL TIME LIMIT and the phrase "CASE TIME LIMIT" will not show up in the problem description.
   So when you get Time Limit Exceeded and find the time your program used is much less than the TOTAL TIME LIMIT, your program must exceeded the CASE TIME LIMIT.
Memory Limit Exceeded (MLE): Your program tried to use more memory than the judge default settings.
Output Limit Exceeded (OLE): Your program tried to output too much. This usually occurs when your program goes into an infinite loop. Currently the output limit is twice the standard output file's size.
Compile Error (CE): The compiler could not compile your program. Warning messages are not considered errors. Click on the link(Compile Error) at the judge's reply to see the actual error messages.
Invalid problem ID: Either you have submitted with a nonexistent problem id or the problem is currently unavailable (probably reserved for coming contests).
System Error: System can't run your program. Such as your program require too many memory (much more beyond the physical memory).

Q: How should I determine the end of input?

A: In most cases there is some information in the input that explicitly indicates the end of input, for example, number of test cases or a single line with a zero following that last test case. But in some cases you have to determine the end of file (EOF) for the end of input. In such cases, you may test for the return value of scanf (which indicates how many values have been successfully read in) or things like cin. Refer to the hints for problem 1001 for a working example.

Q: The time limit is 1000MS. But some guys' programs ran for several seconds to get AC!

A: Please be mercy with those who use Java. As is known to all Java programs run much slower than those written in C/C++. Therefore the time limits for Java programs are longer(exactly five times as much as C/C++). But TLE is still possible for unacceptably slow programs. If such programs are not Java programs, please let us know.

Q: My program exceeds the time limit by just 314MS. How can I improve it?

A: In most cases your program actually requires much more time than the time limit. The judge kills the process instantiating your program once it finds the time limits is exceeded. Usually this occurs at 314MS later than the time limit. If you believe your program really runs for only a little bit longer than the time limit, please submit it a few more times. Possibly error in timing may earn you an AC. And again, AC with efficient code is recommended.

Q: I have more questions.

A: Please make full use of our forum. Post your questions in a nice way. The administrators and other users will try to help you.