Let me explain! My inspiration came from this problem https://mirror.codeforces.com/contest/1811/problem/E, during contest I wasn't able to solve this DIV3_E, but after seeing the solution I was amazed by how easy and beautiful it was.

Basically you have to convert the number in base 9 and map the corresponding digits.(here is my code: (https://mirror.codeforces.com/contest/1811/submission/201234595)

So of course now it seems relay easy, but at first for me, during contest, it wasn't like that at all.

I am sure you have encountered similar problems and I thought it might be interesting to share them.