A way of traversing a tree that processes the current node before processing the left and right subtrees. This is a depth-first algorithm, and can be compared to inorder traversal and postorder traversal.

Generally speaking, this is the most useful of the depth-first searches in terms of search, as a node is processed before either subtree is recursively searched.

Preorder Traversal | Postorder Traversal | Inorder Traversal

1. Visit the root.
2. Traverse the left subtree.
3. Traverse the right subtree.

For those of us who are visual learners, preorder traversal of a binary tree would look something like this.

           A     depth = 0
          / \
         B   D   depth = 1
        /   / \
       C   E   F depth = 2