A BST can be broken by accessing one of its nodes, how can I always make sure this happens?
Hint: In a BST, an inorder traversal produces the keys in sorted order. If we had N.key = $20$ and the inorder traversal was $2, 4, 10, 11, 17, 20, 28, 31, 40$, what $x$ values would break the sorted order when put in the place of $20$?