In this problem, it said "we partition the elements of ...". Here, the word partition should mean that we can only partition the array without shuffling the order of the elements. For example, you can partition [1,2,3,4,5] to {1,2,3} and {4,5} but not {1,3,4} and {2,5}. However, the editorial showed that you can do the latter one. To tell you i'm not misunderstanding the word "partition", here's the translation of this word with Google Translator:

Obviously, the Chinese word "分割" shares the same meaning with what I'm talking above.