I solved all CD problems from Atcoder Beginner Contest #1 to #115. I was able to solve about 70% of the C problems and about half of the D problems on my own. I rarely used special algorithms, and I think they can be solved if you can think through them properly. Here are some useful things to note:
- Try solving the first 1-2 examples by hand. Especially for geometry problems, actually draw them out.
- When brute-forcing multiple parameters, fix one parameter and think about it.
- Pre-compute sorts and cumulative sums in advance.
Additionally, the explanation videos on Youtube are very well done and were quite helpful.