First we need to know the concept of a tree: a tree is a finite set of n nodes.
Degree of a node: refers to the number of subtrees a node has.
The degree of the tree: the maximum degree of the nodes in the tree.
Leaf node: a node with degree 0.
Branch node: a node whose degree is not 0.
The level of the node: starting from the root, the level of the root node is 1, and 1 is added layer by layer.
Depth of tree: The maximum level in the tree.
Ordered tree and unordered tree: It depends on whether we consider it to be ordered. If the elements in the two trees are in the same position, they are considered to be the same; the unordered tree only needs to have the same elements. .
Forest: A collection of disjoint trees.