Speedy interview process: within four weeks, I went from submitting my CV to being rejected.

The process starts with a phone interview, followed by a Skype interview, and then on-site interviews.

The interviews are very focused around C/C++. Despite being told you can use any language, every answer you give will be redirected to a C/C++ question. Know how those languages work internally; focus heavily on that.

The actual problem-solving part was very easy, and I actually found the Skype interview harder than the on-site interviews. Despite solving all their problems, I failed some C/C++ language knowledge questions and was rejected.

They seem to value actual knowledge more than problem-solving abilities, so keep that in mind.

How does GC work in Java?

Compare C/C++ memory management and parameter passing mechanisms.

Actual problem-solving questions are easy and standard. Have a look at Careercup for some examples. They don't really bother changing them, so you might get the same ones as on there.