As you experienced, I have also experienced the same in many companies, seems these people feel pride in making the candidates wait & show themselves busy. I know they may be having work but time of other should also be respected. Regarding the cube root, I am not sure if they were looking some mathematician else why do I need to remember how to calculate cube root or square root. For BST, to find the wrong node, while traversing keep checking if a node is rightly placed or not & I think can be done in O(n)