I'd say the step-by-step hint process is the secret sauce. It mimics an actual interview scenario (where a generous interviewer might be willing to help you along) while teaching by example some structured ways to break down the problem by walking through it step by step. Notably, most of the answers start with a brute-force solution and optimize from there, which (IMO) is an excellent way to approach these sorts of problems, especially in a stressful situation such as a real job interview.