The tree traversal is the process of visiting every node exactly once in a tree structure for some purposes(like getting information or updating information). In a binary tree there are some described order to travel, these are specific for binary trees but they may be generalized to other trees and even graphs as well.

Preorder means that a root will be evaluated before its children. In other words the order of evaluation is: Root-Left-Right

Preorder Traversal
    Look Data
    Traverse the left node
    Traverse the right node

Example: 50 – 7 – 3 – 2 – 8 – 16 – 5 – 12 – 17 – 54 – 9 – 13

Inorder means that the left child (and all of the left child’s children) will be evaluated before the root and before the right child and its children. Left-Root-Right (by the way, in binary search tree inorder retrieves data in sorted order)

Inorder Traversal
    Traverse the left node
    Look Data
    Traverse the right node 

Example: 2 – 3 – 7 – 16 – 8 – 50 – 12 – 54 – 17 – 5 – 9 – 13

Postorder is the opposite of preorder, all children are evaluated before their root: Left-Right-Root

Postorder Traversal
    Traverse the left node
    Traverse the right node 
    Look Data

Example: 2 – 3 – 16 – 8 – 7 – 54 – 17 – 12 – 13 – 9 – 5 – 50

class Node:
    def __init__(self,key):
        self.left = None
        self.right = None
        self.val = key

def printInorder(root):
    if root:
        printInorder(root.left)
        print(root.val)
        printInorder(root.right)

def printPostorder(root):
    if root:
        printPostorder(root.left)
        printPostorder(root.right)
        print(root.val)

def printPreorder(root):
    if root:
        print(root.val)
        printPreorder(root.left)
        printPreorder(root.right)