def test_invalidTreeIsNotBST(self):
root = TreeNode(20)
root.left = TreeNode(10)
root.right = TreeNode(30)
root.right.left = TreeNode(5)
root.right.right = TreeNode(40)
self.assertFalse(is_bst(root))
def test_validBinaryTreeIsValid(self):
root = TreeNode(5)
root.left = TreeNode(3)
root.right = TreeNode(7)
root.left.left = TreeNode(1)
root.left.right = TreeNode(4)
root.right.left = TreeNode(6)
self.assertTrue(is_bst(root))