How to calculate the depth of a binary search tree

Question

I would like to calculate the summation of the depths of each node of a Binary Search Tree.

The individual depths of the elements are not already stored.

Answer 1

For any given tree, the number of nodes is 1 for the root plus the number of nodes in the left subtree plus the number of nodes in the right subtree :)

Details, like making sure there actually is a left or right subtree, are "left to the reader".