Sometimes I stuck in div2 B and can solve C easily than B. Is it important to practice 800-1300 rated problems also to maintain the speed? How many those problems need to be practiced daily? What is your strategy?

Problem link I used dijkstra's algorithm and when I get a new bicycle with minimum slowness factor or when I reached a city with minimum time I updated the distance array. This was my submission. But it didn't work. I saw many people use 2d distance array. But I can't figure out the intuition? Could you explain me the intuition. If there is any other way to do this could you comment that one too.

You are given an string s of length n. And there is a eraser and it deletes neighboring characters of a particular letter when it place on a specific position. For an example 'aaaaba' when we place eraser on 1st 'a'(index 0), string becomes 'ba' (since we deleted all the letters of the neighboring 'a's). Task is to delete the entire string in minimum operations.

Test 1
Input -> 6 aabcbb
Output -> 3

Constraints
1<=n<=1000

I would like to know what topics that I need to master and which difficulty level problems that I need to do in order to become a specialist. Currently I'm doing 1400 ones and if I will do 1600 ones is that will be good? Thank you.

Take a loot at the following image. Suppose you have to find out the answer for the y. So how to find that answer. I used binary search but it didn't work.

Which topics that I have to master in order to become a pupil? And from which difficulty level problems that I have to do? Is it good to do problems from 1300-1600?