Error in 6th Edition
Maybe you fixed this already in 7th.
In 6th edition, question 4.11, the question only specifies a binary tree but the answer assumes a binary search tree. Which is exactly a thing you told us to look out for. So I was.
EDIT: Although this doesn't really seem to affect the answer.
Also, for the love of all things holy, invest in some spam-blocking on the forums!