They will likely present scenarios to you, and you will write pseudo-code (verbal or on a whiteboard) to walk through solving the problem.
A simple one might be something like, 'what day of the week is it based on a specific date' or 'you have a list of users with their birth dates and address, how can you organize localized birthday parties'
I've never done one, but I've been around these guys long enough to know how they think...answers leaning towards 'rent-a-coder.com' or 'google it' don't usually go over well
They want to know how you think and problem-solve complex calculations; the ones above are super simple....think of some more elaborate ones and answer them yourself; this is is good mental exercise in general.