I am kinda going through the same thing. I recommend Cracking the coding interview very much. The thing I am doing is watching youtube videos on a particular topic (say linked lists) to get the basics right. Write some pseudocode and walk through it to make sure my logic is right and all. Then jump on to the problems. Also, my advise would be not to jump to solutions unless you are really stuck and cannot think of anything else. I am trying to solve them using brute force technique if nothing else. Then try to improve on that. My prep time for each topic depends on the complexity. For example I have been studying trees since a week.