The description clearly states the second example is a valid BST which it isn't or am I incorrect in that statement?

Furthermore,
' you will write a function that will validate that a given binary tree is a binary search tree. '
please provide a reference to a binary search tree definition that states a reverse order is a valid binary search tree.

Second Example is NOT A VALID BST! and the corresponding test is incorrect

by definition, valid BST's have the following properties :

• The left subtree of a node contains only nodes with values lesser than the node’s value.
• The right subtree of a node contains only nodes with values greater than the node’s value.
• The left and right subtree each must also be a binary search tree.

Although the tree traversal meant to be used here is not inherently a BST property, its output is, which is the key to the solution.

what is incorrect? can you elaborate?

Incorrect.

this is not a BST problem. it is a general BT problem. why put a misleading title?

C++ translation

Please, review and (hopefully) approve.

AWSOME ONE, MUCH BETTER THAN THE LEECODE VERSION! THE AUTHOR IS ABSOLUTE A GENIUS!

Done ;-)

Fortran 2008 should be enabled...

It seems to me that random tests occasionally include duplicate numbers what is explicitly mentioned in the task description as not happening... :-(

I feel like the most common definition of a BST has all of the left nodes <= the root node and all right nodes > the root node (ascending order). Allowing descending order BSTs is a nice twist but I think it should be mentioned in the instructions.