It is 2016 and therefore Facebook started its Facebook Hacker Cup 2016. It has some challenging exercises which you need to solve in a few hours. But you also need to write efficient code. If you don’t do that, you will not have enough time to solve an exercise and points will be lost. In this article, I will highlight the “High Security” exercise in the Facebook Hacker Cup qualification round. I will first give the problem description which was posted on Facebook and then tell how I tackled the problem.