Project Euler is the lifehack for job interview prep.

Find problems that are appropriate for your skill level, write out answers on paper or on a whiteboard, then once you're happy with them, put them into the computer and fix them up until they run. Keep doing that until you feel well-practiced with your ability to put code together without having an editor and think quickly about new problems you've never seen before. (For me it took a few days of practice.) Congratulations, you're now the guy or gal who can show up and eat the interview problems alive and look comfortable and confident doing it.