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:

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 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 leaf (3, 6, 8, 10, 12, 14, 18, and 25 are leaves).

A node can have ancestors and descendants. The ancestors of a node (except the root) are parent, grandparent, great-grandparent, and so on. The descendants of a node are child, grandchild, great-grandchild, and so on. For example, node 5 has 7 and 11 as its ancestors and 3 and 6 as its descendants.

Another terminology used with trees is the subtree. A subtree consists of a node and its descendants. For example, nodes 13, 12, and 14 consist a subtree...