It has occured to me while I am solving more questions (20 Q/day), I am forgetting how to solve those questions I learned to solve 2 weeks ago. What is the approach to learn, but not to forget. My goal is to solve 1000 Question by end of next month, so if there is any question asked in interview, I can recall the question.

I am not "memorizing" the solutions. However, I had to look at the solution because my mind was blank. What is the idea to remember the approaches.