Thoughtworks Software Developer interview questions

How would you explain to your friend, who does not know nothing about software engineering, what exactly a Software Engineer does?

It was a programming task from scratch. They literally wanted me to code some graphing algorithm in Java to find the shortest possible path in a given graph. The Java program I supposed to write had to have a good object oriented design, had to read the graph from a file, had to create a graph out of it and it had to solve the problems provided in an efficient way. It took me 2 days to finish up the fully working Java code.