Recently, I came across this problem through the USACO Guide (thanks Benq, thecodingwizard, caoash, darren_yao, nchn27, and frodakcin for the the amazing resource!).

I know (from the guide) that the solution involves Wavelet Trees, I have no idea how to use it here (actually, I really only understand the basic implementation).

Any hint/solution/approach is greatly appreciated.

Thanks.

Edit: I couldn't find any editorials online either, the closest I found was this. The problem comes from the 2019 ICPC Asia Danang Regional Contest (Problem E).