Usually, when people prepare for programming interviews, they do exercises on a website called LeetCode: https://leetcode.com/
The kinds of questions asked in entry-level software engineering interviews typically fall under the "algorithms" category. And unless the interview is for a senior position, the questions will be of the "Easy" to "Medium" difficulty. Based on the description of your interview, it sounds like your programming exercise will be on the easier end.
> Ranges are something I hadn't thought of. That's a good idea - but I'm not too sure I'm looking for that level of sensitivity, if that makes sense.
That's fine. Just something to consider.
I'm curious about your project now--what do you hope to get from sentiment analysis on certain subreddits?
If you still want to create a sentiment corpus from subreddits, you could use Amazon Mechanical Turk to help you with sentiment tagging "by hand". The idea is that people earn money by performing tasks that you set up (e.g. answering questions). You can provide text fom your corpus and ask people to pick a sentiment that best matches the text. Of course, you will have to pay for the service. In addition, people may "cheat" the system in order to make more money while providing random answers (e.g. choosing "negative" for each scenario), but you could remedy that with smoothing.
Also, looking online, I found an article that may help with your project:
http://www.academia.edu/3525454/Predicting_sentiment_of_comments_to_news_on_Reddit
Well, my advice would be to go to your uni's library and have a look for some books on language technology/computational linguistics. Although I come from the U.S and lived there most of my life, I now live and am doing my master's in Germany and so I unfortunately I don't know much about any English-language books to get you started with an introduction to the field, however, I have heard this book get thrown into a few conversations in classes:
http://www.amazon.com/Handbook-Computational-Linguistics-Language-Processing/dp/1118347188
I am also pretty sure your uni library will have something on it in the linguistics/compsci section. However, if this isn't the case, send me a PM and I will try to help you out. I have a ton of information that I could simply "translate" into English and give to you if you'd like to know where to get started with research for it and such.