Here’s the video and code for the Calendar Conflicts video which walks through a linear complexity solution to a common coding interview problem. The idea is that you’re given a list of calendar events and you must find any events in the calendar which conflict.
There is an easy brute force solution to this problem, but the optimal solution is linear. The code here is written in vanilla PHP. If you’re interviewing at a company like Google, Facebook, Apple, etc., this is exactly the kind of coding interview question you might have to face.