The interview process begins with a brief phone call with HR to gather information about the company, career options, and gauge your interest.

Next, you'll complete a HackerRank coding test featuring two problems with a three-hour time limit. This means you need to pass most, if not all, unit tests, which are not published. While most tests focus on correctness, at least one tests performance and will fail a brute-force, slow implementation.

If the coding test is successful, a technical phone interview follows. This typically includes a brief introduction to Two Sigma, a series of technical questions, and an opportunity for you to ask questions. Knowing or remembering CS basics such as O(n) complexity, hashing, trees, and design patterns is helpful.

After the phone interview, you'll be invited for a full day of in-person interviews. The morning consists primarily of technical interviews. One interview may involve sitting at a computer with an interviewer to fix a broken implementation and write unit tests using an IDE like Eclipse. In another, you might design a game on a whiteboard and then implement it in Eclipse.

Following the morning technical sessions, you'll be taken out to lunch while the team discusses your performance and whether to proceed with afternoon interviews. If invited back, you'll have three further interviews with managers in areas for which you are a potential fit. These questions tend to be more high-level.

A fourth discussion with HR will cover the next steps. After a few days, HR may contact you with good news, expressing interest in onboarding you but requesting further information. This might lead to another interview, preliminary compensation discussions, and eventually the offer package.

To prepare for this and many other technical interviews, the book "Cracking the Coding Interview" is highly recommended. It covers forgotten CS basics and offers valuable advice on preparation, test-taking, and design. A strong understanding of unit tests will also be beneficial.

I've found the Two Sigma HR personnel to be helpful and responsive. They are rapidly expanding their employee count, which I'm sure contributes to certain inefficiencies others have reported.

Hashtable implementation Thread vs. process Finding the median of a stream of numbers Secure generation of passwords Design a simple game Fix a coding problem using unit tests