# Postorder tree traversal in Python

In this tutorial, we will learn about the Postorder traversal of a tree data structure in Python. It is one of the three variants of the Depth-first search. Postorder tree traversal is also known as the LRN algorithm (Left, Right, Node). In the previous tutorials, I have also explained about Inorder traversal and Preorder traversal in Python, which are the other two variants of DFS.

Let’s create a binary tree and learn to implement this algorithm. Since we are using the recursive method to traverse the tree, I recommend you to get familiar with recursion.

class Tree: def __init__(node,value): node.value = value node.right = None node.left = None def create_node(node,value): if (node.value is None): node.value = value else: if( value < node.value ): if (node.left is None): node.left = Tree(value) else: node.left.create_node(value) elif( value > node.value ): if ( node.right is None): node.right = Tree(value) else: node.right.create_node(value) Root = Tree(5) Root.create_node(7) Root.create_node(2) Root.create_node(3) Root.create_node(6) Root.create_node(1) Root.create_node(8)

## Postorder traversal of a Tree in Python

def Postorder( node, Root ): if( Root is None ): return node.Postorder(Root.left) node.Postorder(Root.right) print(Root.value,end = ' ')

- Traverse the left subtree recursively.
- Traverse the right subtree recursively.
- Access the value of the current node.

The order of the Postorder traversal is **1 3 2 6 8 7 5**.

Explanation:

- Firstly we created a binary tree with eight nodes and traversed using a postorder tree traversal algorithm.
- If the node is None, the control will return back to the calling function. Since we created a binary tree, the node is not None. As a result, we traverse the left subtree and the last left node value is printed. After that, we traverse the right subtree and print the values. At last, the parent node is printed.

#### Here is how the complete code should look like

class Tree: def __init__(node,value): node.value = value node.right = None node.left = None def create_node(node,value): if (node.value is None): node.value = value else: if( value < node.value ): if (node.left is None): node.left = Tree(value) else: node.left.create_node(value) elif( value > node.value ): if ( node.right is None): node.right = Tree(value) else: node.right.create_node(value) def Postorder( node, Root ): if( Root is None ): return node.Postorder(Root.left) node.Postorder(Root.right) print(Root.value,end = ' ') Root = Tree(5) Root.create_node(7) Root.create_node(2) Root.create_node(3) Root.create_node(6) Root.create_node(1) Root.create_node(8) print('Postorder traversal :',end = '') Root.Postorder(Root)

**Output:**

Level order traversal :1 3 2 6 8 7 5

If you have any quires, please drop in your comments.

Thank you…😊

thanks for the post. Learned something new from it.