You can ignore any need to sanitize words or add punctuation

So it is guaranteed that there wont be spaces inside the words

If you use String.split you create whole array in memory.
Consider using Pattern.splitAsStream to process one word at a time.

dito

same here. I don't understand it.

There should be at least on test to make sure the provided code doesn't violate "Be careful, there shouldn't be a space at the beginning or the end of the sentence!"

could someone explain (in other words) what "but displaced, or rotated, by any number of steps." means? I really don't get it :)