Binary tree

1. Definition

     A binary tree is a tree structure in which each node has at most two subtrees . Usually a subtree is called “ The left subtree ”（left subtree） and “ Right subtree ”（right subtree）.

     Perfect binary tree ： In a binary tree , Except for the last floor , If the rest of the layers are full , And the last layer is either full , Or there is a lack of consecutive nodes on the right .

2. Simple concept

The degree of node ： The number of subtrees owned by the node .
leaf ： A node of degree zero .
Branching nodes ： Nodes with degree not zero .
The degree of a tree ： The maximum degree of a node in a tree .

level ： The level of the root node is 1, The hierarchy of the other nodes is equal to the hierarchy of the parent nodes of the node plus 1.
The height of the tree ： The maximum level of nodes in a tree .
Disordered trees ： If the order between the subtrees of the nodes in the tree is not important , You can switch places .
Ordered trees ： If the order between the subtrees of nodes in a tree is important , You can't swap places .
The forest ：0 One or more disjoint trees make up . Add a root to the forest , The forest becomes a tree ; Delete the root , Trees become forests .