Implementation of binary tree and four kinds of common traversal (python) __python

Constructing a Node class

Constructs the tree class and increases the method of adding nodes

Pre-sequence traversal (root, left, right), sequence traversal (left, Root, right), sequential traversal (left, right, root)

Using queues to achieve breadth-first traversal, that is, hierarchical traversal

There are two main types of traversal of the reference tree, one is the depth-first traversal, the first order, the sequence, the second, and the other is the breadth-first traversal, like the hierarchical traversal. The difference between the two is not very obvious in the tree structure, but the efficiency and function of depth-first search and breadth-first search are very different when we extend from the tree to the direction graph and to the direction-free graph.
The depth first generally uses the recursion, the breadth first generally uses the queue. Most of the algorithms that can be implemented by recursion can also be implemented using stacks.

My impression is that there is a recursive structure of the tree method, but I have not been thinking how to construct. After a careful thought, recursive thinking is somewhat similar to depth-first algorithm, and tree construction should be breadth first. If you use recursion, you must have a termination condition, such as a rule tree depth. Otherwise, the tree will be constructed in favor of the left tree or the right list tree. Therefore, the construction of the general tree or should be used in the queue better.