Binary tree is defined as a connected acyclic graph in graph theory, and the degree of each vertex is not greater than 2. A rooted binary tree must satisfy that the degree of the root node is not greater than 2. For the root node, each vertex defines a unique parent node and at most two child nodes. However, there is not enough information to distinguish the left node from the right node. If connectivity is not considered, multiple connected components are allowed in the graph, and such a structure is called a forest. A forest can also be regarded as many non-empty trees, and only one tree can be regarded as a forest. But the forest is not necessarily a tree. The forest can also have roots, when every tree in the forest has roots.

My understanding is: Binary tree is a very special forest;