I'm solving 50 problems at each level. Recently I moved up to 1500 rated problems. I realize that the problems in this level are significantly different from 1400 rated problems topic wise. Till 1400, problems could be done by vectors mostly, but now, there's dp and trees.

Can anyone share with me if there is any theory I must read to get through this level or should I just learn the concepts through editorials of the problems that come forth?