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

Something like this:

int countChildren(Node node)
{
    if ( node == null )
        return 0;
    return 1 + countChildren(node.getLeft()) + countChildren(node.getRight());
}

And to get the sum of the depths of every child:

int sumDepthOfAllChildren(Node node, int depth)
{
    if ( node == null )
        return 0;  // starting to see a pattern?
    return depth + sumDepthOfAllChildren(node.getLeft(), depth + 1) + 
                   sumDepthOfAllChildren(node.getRight(), depth + 1);
}

Now for a hopefully informative explanation in case this is homework. Counting the number of nodes is quite simple. First of all, if the node isn't a node (node == null) it returns 0. If it is a node, it first counts its self (the 1), plus the number of nodes in its left sub-tree plus the number of nodes in its right sub-tree. Another way to think of it is you visit every node via BFS, and add one to the count for every node you visit.

The Summation of depths is similar, except instead of adding just one for each node, the node adds the depth of its self. And it knows the depth of its self because its parent told it. Each node knows that the depth of it's children are it's own depth plus one, so when you get the depth of the left and right children of a node, you tell them their depth is the current node's depth plus 1.

And again, if the node isn't a node, it has no depth. So if you want the sum of the depth of all the root node's children, you pass in the root node and the root node's depth like so: sumDepthOfAllChildren(root, 0)

Recursion is quite useful, it's just a very different way of thinking about things and takes practice to get accustomed to it