The Tree such as that root is 1,left child of root is 2,right child of root is null can be accept.However,according to the topic,we can only see 1,And 2 is the left tree of root tree 1.

Sorry, I have change the ambiguous expression of my question.root is 1,left child of root is 2,right child of root is null,according to the topic, I think result is [1],not [1,2],but [1,2]is accept