DSA Tutorial

POST ORDER TRAVERSAL

🌸 Postorder Traversal in DSA (Binary Tree)

Postorder traversal is a depth-first traversal method where nodes are visited in the order:

Postorder Order: Left → Right → Root

🌳 Example Tree

➤ Postorder Output: 4 5 2 3 1

🧠 Postorder Traversal Algorithm

Recursively traverse the left subtree.
Recursively traverse the right subtree.
Visit the root node.

💻 Python Code

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

def postorder(root):
    if root:
        postorder(root.left)       # Left
        postorder(root.right)      # Right
        print(root.val, end=" ")   # Root

⚙️ Output

➤ 4 5 2 3 1

🌐 Real-Time Use Case

📌 File system deletion: Postorder is useful when you need to delete a directory — delete files (children) first, then the directory (parent).

🔄 Iterative Postorder Traversal (Using Two Stacks)

Postorder can also be done without recursion using two stacks:

def postorder_iterative(root):
    if root is None:
        return
    s1, s2 = [], []
    s1.append(root)
    while s1:
        node = s1.pop()
        s2.append(node)
        if node.left:
            s1.append(node.left)
        if node.right:
            s1.append(node.right)
    while s2:
        print(s2.pop().val, end=" ")

✔️ Tip: Postorder is the most natural traversal for tasks like deleting trees or evaluating expressions from expression trees.

📌 Summary

Traversal Order: Left → Right → Root
Used in deletion, cleanup, and evaluation problems.
Can be implemented both recursively and iteratively.

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