I am sorry -- since Gayle's book of the same name is a product worth ~$30, I assumed there was a paywall at some point behind https://www.hackerrank.com/domains/tutorials/cracking-the-co... -- I guess I was wrong? It doesn't clearly say on the page that it was, and I don't want to sign up for their product.

A slightly weaker version of that argument is that having a vast base of signed-up users and data about their challenges is still making bank out of the efforts of individual users, but clearly not as bad as requiring you to pay for this tutorial.