How to build software that mimics topcoder, codingbat etc.?
<p>I want to build a downloadable desktop software which shows the user a GUI with a sample problem.</p>

<p><img src="http://i.stack.imgur.com/mxbww.png" alt="enter image description here"></p>

<p>The user then proceeds to code a solution and pastes it to a blank screen.</p>

<p>The program evaluates it and gives points to the user. The user progresses to the next level.</p>

<p>My question is, how do these sites evaluate a program for correctness? I don't mean to measure performance. </p>

<p>Do they write their own compiler or parse the text and feed it to javac and extract the output as well as search for some required keywords in the code.</p>
