I was referred by a friend.

The whole process took a week and a half, primarily due to a pending deadline on my part.

I had a phone interview and was then invited onsite at the end of the week. The onsite interview was composed of three interviews.

They asked typical questions found in most technical interviews, although they were slightly more tree-heavy than at other companies.

On Monday, I was contacted asking to provide references. I thought I had it in the bag because of this, but on Tuesday evening, I was rejected.

I know my references were strong and wouldn't have caused my rejection. So, I guess references are asked for before the onsite interviews are processed?

I signed an NDA, so I won't disclose the exact questions.

However, I'd practice up on tree traversals before interviewing.