** Trees**

A * tree* is a kind of * graph*, composed of * nodes*
and * links*, such that:

- A link is a directed pointer from one node to another.
- One node, called the
*root*, has no incoming links. - Each node, other than the root, has exactly one incoming link from its
*parent*. - Every node is reachable from the root.

A node can have any number of * children*. A node with
no children is called a * leaf* ; a node with children is
an * interior node*.

Trees often occur in computer systems and in nature.