Have been fairly consistent in solving div2 C across the last few contests. Have never solved a Div2 D during contest(ever). Any and all advice (topics required, curated problem sets, courses) is most welcome. Hope to reach expert soon :D

Question : here

Submission 1 (TLE) : 293404599, used vector of size 2*1e5 for frequency storage for future lookup

Submission 2 (passed) :293423236, used map for same purpose

Can someone explain why I encountered TLE in the first case?

To go from pupil to specialist, my plan is to clear the third question of upcoming Div2 contests(with less priority on time and more on clearing the question itself). However, as I am mostly not able to clear the third question even with sufficient time during a contest(So far, I have been able to clear A and B), I wanted some advice from the more experienced members on codeforces, as I am starting to believe that my current method of preparation is not the most effective...

Specifically these questions mainly:

1. What are the most common topic tags and difficulty level tag for d2c, and an effective practice regime to prepare for them?
2. Any bonus tips are also appreciated.
3. Also any tips to clear A and B faster is welcome. 4.(surely, this question must have been asked before, but I have not found anything relevant yet, so if anyone can link a blog regarding answers to the same, that is also cool)