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TERMINOLOGY

• A tree is an abstract model of hierarchical structure.
• It consists of a finite set of elements, called nodes, and a finite set of directed lines, called branches, that connect the nodes.
• The number of branches associated with a node is the degree of the node. When the branch is directed towards the node, it is an indegree branch; when the branch is directed away from the node, it is an outdegree branch.
• If the tree is not empty, then the first node is called the root.
• A leaf is any node with an outdegree of zero.
• Nodes that are not a root or a leaf are known as internal nodes.
• A node is a parent if it has successor nodes.
• A node with a predecessor is a child.
• Two or more nodes with the same parent are siblings.
• Ancestor is any node in the path from the root to the node.
• A descendant is any node in the path below the parent node.
• A path is a sequence of nodes in which each node is adjacent to the next one.
• The level of a node is its distance from the root.
• The height (depth) of the tree is the level of the leaf in the longest path from the root plus one.
• A subtree is any connected structure below the root.