Nice!

There is no built-in SortedList

For what it's worth PriorityQueues are about as close as you get to a sorted List if anyone is ever in need of Queue-only functionality but wants it sorted.

I actually find it annoying/confusing that String.chars() returns an int stream of char integer values

This really frustrates me too. I haven't seen a good argument for why there aren't CharStream, FloatStream, etc. Creating special *Streams for only some primitives is an injustice (and creates weird scenarios like this where java.* API returns chars as an IntStream)

Your points on Character vs Number are well-taken, while both routes employ some confusion, I think I probably agree with you more than myself from yesterday now.

Also, since the most straightforward way to recombine digits is via Long.valueOf(String), I am more confident leaving my digits as Characters as a reminder that we are going to come back to String.

Character.getNumericValue(char) might be even more straightforward (unfortunately I found it after completing my own solution).

The return value is definitely confusing but I tend to give a pass to java.* API when it is confusing and use it anyways.

spoiler flag, plz...

spoiler flag, plz...