in the name of god

i wonder which algorithms and data structures needed to solve D and E questions

i can solve C questions

but i can't solve D and E questions and i wonder what must i focus on it

actually practice and practice more is main way but i don't know that i must practice C questions more and more or i must

practice hard questions and one thing more is that how must i practice hard questions

for example think on one question for a weak or think on it 2-3 hours then read editorial of it...