They sent an email: "You will have 90 minutes to complete this challenge, using either Java or C++ programming languages. As you cannot log back in once you have started, please ensure that you complete the challenge in one sitting, while also ensuring you only work within the single tab the test is open in. Should you do well, we will move you onto the next step in our interview process, which typically consists of 2-3 technical phone interviews with our engineers."