Need help in understanding this codeforces editorial

Can someone give me a more detailed explanation of technique used in the solution to problem E ? Or maybe post a link where it is explained in more detail.

I searched on codechef and found this but this also links to the codeforces editorial. There is also a link to some slides but the slides are not there.

A broad, step-by-step overview of the algorithm would also help.


link to slides.