Online Judge

Introduction to Online Judge

For programming contests, you have to prepare yourselves by practicing and solving problems. At any particular contest, a certain number of problems are set. Usually the time limit to solve 8-10 problems in 4-5 hours. If you practice a lot of problems beforehand, you'll be able to solve a good number of problems.

There are a large number of online judges, where you'll find a huge number of problems to solve and check if your solution is correct.

 According to Wikipedia, the free encyclopedia, "An online judge is an online system to test programs in programming contests. They are also used to practice for such contests. Many of these systems organize their own contests.
The system can compile and execute codes, and test them with pre-constructed data. Submitted code may be run with restrictions, including time limit, memory limit, security restriction and so on. The output of the code will be captured by the system, and compared with the standard output. The system will then return the result. When mistakes were found in a standard output, re-judgement using the same method must be made.
Online Judges have rank lists showing users with the biggest number of accepted solutions and shortest execution time for a particular problem."


Let's get know some well known online judges and a complete reference guide how to use online
judges.


UVa Online Judge (http://uva.onlinejudge.org/)

The name that comes first with online judges, is the UVa Online judge. It one of the largest online judge having at least 5000+ problems and best for beginners.

Here you will find hundreds of problems. They are like the ones used during programming contests, and are available in HTML and PDF formats. You can submit your sources in a variety of languages, trying to solve any of the problems available in their database.
You can see Contest Rankings section at the Live Rankings link, use the new Quick access, info and search option on the left menu for and easier navigation. (The tool will be updated next days for a more complete information.)

Check UVa today, register there and start solving problems.

Top Coder (http://www.topcoder.com/reg/)

 
It is are a community of more than 455,000 software developers, algorithmists, and digital designers. The community is accessed via the TopCoder Platform, which is the most advanced and sophisticated productivity and innovation platform in the world.

They arrange regular online contests and you can even earn cash prizes from here. It is one of the most respected platforms for programmers.


TopCoder is the world’s largest platform for digital open innovation. They provide the platform and a community of over 445,000 global members to accelerate the development of new digital products and services for our clients – Fortune 1000 enterprises, small and mid-sized businesses and government agencies.

Register in TopCoder from here: http://www.topcoder.com/reg/

Download Java Applet for Top Coder Arena from here, then login with your topcoder and password: 

http://community.topcoder.com/contest/arena/ContestAppletProd.jnlp

CodeForces (http://codeforces.com/)

Codeforces is a project joining people interested in and taking part in programming contests. On one hand, Codeforces is a social network dedicated to programming and programming contests. On the other hand, it is a platform where contests are held regularly, the participant's skills are reflected by their rating and the former contests can be used to prepare. Codeforces constantly develops and we plan to improve the platform to give the participants the opportunity to organize their own contests, filling the project with learning content, developing Codeforces as a training and learning platform.

Contests are regularly held on Codeforces. Participating in them is free and open to everybody. Every month we organize approximately six contests. To participate, you have to be registered on the site (if you have an OpenID or a Gmail account, then you won't even have to memorize the password) and register for the oncoming contest. Make sure that you are present in the list of the users, registered for the contest, before the registration ends. Usually, if you can't take part in the contest officially (e.g. if it's the contest for the second division and you are in the first one), then you can register for the contest to participate out of competition.
Register CodeForces from here: http://codeforces.com/register

LightOJ (http://www.lightoj.com/)

This is first and only Bangladeshi online judge. Here you'll get over 534 categorized problems. Here you can arrange your own contest after solving 25 problems. You can even set your own problems.
Register here: http://www.lightoj.com/register_user.php

Some other online judges are listed here. Check them yourself:

• acm.timus.ru/
• www.spoj.com/
• poj.org/ 
• acm.tju.edu.cn/ 
• www.urionlinejudge.com.br/ 
• judge.u-aizu.ac.jp/ 
• coj.uci.cu/ 
• acm.sgu.ru/

So, wat's up? Register to any of these online judges and start solving problems NOW! If you are a complete beginner, I would suggest you to register in UVa Online Judge.
Any question?
Comment here, I would like to answer.
Shahriar, 22-4-2013