I can only solve 3 problems in Div.2 all the time. I leant dp, trees and graphs, but I still can't solve Div.2 D by myself. Because of that, I can't get higher rating. Can anybody help me? I am stuck at 1600 ~ 1700.

Update on 2024/8/15: One year has passed. I can solve problem D by myself sometimes. I tried solve problem E too when I am practising but hm I am still an expert. I may make mistakes in some contests (fail to solve problem C). What shall I do?