A tree consists of nodes with a parent-child relationship. Each node has a parent (except for the first node at the top) and zero or more children, as in the following figure:
The top node of a tree is called the root (11). It is the node that does not have a parent. Each element of the tree is called a node. There are internal nodes and external nodes. An internal node is a node with at least one child (7, 5, 9, 15, 13, and 20 are internal nodes). A node that does not have children is called an external node or a leaf (3, 6, 8, 10, 12, 14, 18, and 25 are leaves).
A node can have ancestors and descendants. The ancestors of ...
