Improving your Programming Skills

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, rejudgement using the same method must be made.

Online Judges have rank lists showing users with the biggest number of accepted solutions. Bunch of resources for improving programming skills using online judges:

http://codingbat.com/

Online code practice which shows the right solution straight away. The best part which is that you can type your code into the text pad on website and see its output

http://livearchive.onlinejudge.org/

Here you will find hundreds of problems used in the ACM-ICPC Regional's and World Finals. You can submit your sources in a variety of languages.

http://uva.onlinejudge.org/index.php

UVa Online Judge is an online automated judge for programming problems hosted by University of Valladolid. Its problem archive has over 3500 problems and user registration is open to all users. There are currently over 100000 registered users. A user may submit a solution in C, C++, Java or Pascal. 

http://www.spoj.pl/

SPOJ – Sphere Online Judge – is a problem set archive, online judge and contest hosting service accepting solutions in many languages.

http://www.topcoder.com/

TopCoder is a company which administers contests in computer programming. TopCoder hosts fortnightly online competitive programming competitions — known as SRMs or "single round matches" — as well as weekly competitions in design and development. 

http://projecteuler.net/

Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.

http://www.codechef.com/

Participate in programming competitions, practice online questions, participate in online discussions.

Reference: http://jugnu-life.blogspot.in/2011/12/websites-for-improving-programming-and.html