Definition A tree is a collection of nodes. The collection can be empty; otherwise, a tree consists of (1) a distinguished node r, called the root (2) and zero or more nonempty(sub)trees T1,., Tk, each of whose roots are connected by a directed edge from r Note Subtrees must not connect together. Therefore every node in the tree is the root of some subtree There are N-1 edges in a tree with N nodes > Normally the root is drawn at the top【Definition】A tree is a collection of nodes. The collection can be empty; otherwise, a tree consists of (1) a distinguished node r, called the root; (2) and zero or more nonempty (sub)trees T1 , , Tk , each of whose roots are connected by a directed edge from r. Note: ➢ Subtrees must not connect together. Therefore every node in the tree is the root of some subtree. ➢ There are edges in a tree with N nodes. ➢ Normally the root is drawn at the top. N − 1