Make sure you do the algorithm *yourself*, and practice, practice, practice until you get it with ease!
http://www.freecodecamp.com/challenges/
On this page of the site you can watch the video online [Walkthrough] - SYSTEMIC DIFFERENCE (recursive solution) | FCC Advanced Algorithms with a duration of hours minute second in good quality, which was uploaded by the user CodeWithNick 14 June 2016, share the link with friends and acquaintances, this video has already been watched 282 times on youtube and it was liked by 3 viewers. Enjoy your viewing!