Postorder Traversal in Binary Tree in java

March 17, 2026

Postorder Traversal in Binary Tree

Postorder Traversal in Binary Tree in Java is a Depth First Traversal (DFS) technique where nodes are visited in the following order:

Left → Right → Root

It is also called LRN traversal. In this traversal, the root node is processed after both left and right subtrees, making it especially useful for problems like tree deletion, expression tree evaluation, and postfix expression generation.

What is Postorder Traversal?

In postorder traversal, we:

Traverse the left subtree
Traverse the right subtree
Visit the root node

Steps to Find Postorder Traversal

Here are some of the steps to find postorder traversal :

Step 1: Print the left most child of left subtree of binary tree i.e 20.
Step 2: Now , before printing the root node, move to right sub-tree and print the left child i.e. 40.
Step 3: Print 50 which is right child.
Step 4: Now, print it’s root node 30.
Step 5: At last print the root of the tree i.e. 10.

The printing sequence will be 20,40,50,30,10.

Example of Postorder Traversal:

Methods for Postorder Traversal in Binary Tree in Java

Postorder traversal in Binary tree can be performed using 2 methods using Java:

Method 1: Recursive Postorder Traversal
Method 2: Iterative Postorder Traversal (Using Stack)

Inorder Tree Traversal in Binary Tree

Preorder Tree Traversal in Binary Tree

Inorder Tree Traversal without Recursion

Preorder Tree Traversal without Recursion

Postorder Tree Traversal without Recursion

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Methods for Postorder Traversal in Binary Tree

Method 1: Recursive Postorder Traversal

Algorithm for Postorder Traversal

postorder(node)

1. If node is null
      return
2. Call postorder(node.left)
3. Call postorder(node.right)
4. Print node.data)

Java Code:

Run

class Node {
    int data;
    Node left, right;

    Node(int value) {
        data = value;
        left = right = null;
    }
}

public class PostorderTraversal {

    Node root;

    void postorder(Node node) {

        if (node == null)
            return;

        postorder(node.left);
        postorder(node.right);
        System.out.print(node.data + " ");
    }

    public static void main(String[] args) {

        PostorderTraversal tree = new PostorderTraversal();

        tree.root = new Node(10);
        tree.root.left = new Node(20);
        tree.root.right = new Node(30);
        tree.root.left.left = new Node(40);
        tree.root.left.right = new Node(50);

        System.out.print("Postorder Traversal: ");
        tree.postorder(tree.root);
    }
}

Output:

Postorder Traversal: 40 50 20 30 10

Method for Postorder Traversal in Binary Tree

Method 2: Iterative Postorder Traversal (Using Two Stack)

Algorithm for Iterative Postorder Traversal

1. Create two stacks: stack1 and stack2
2. Push root to stack1
3. While stack1 is not empty:
       a. Pop node from stack1 and push to stack2
       b. Push left child to stack1
       c. Push right child to stack1
4. Print elements from stack2

Java Code:

Run

import java.util.Stack;

class Node {
    int data;
    Node left, right;

    Node(int value) {
        data = value;
        left = right = null;
    }
}

public class PostorderIterative {

    Node root;

    void postorder() {

        if (root == null)
            return;

        Stack stack1 = new Stack<>();
        Stack stack2 = new Stack<>();

        stack1.push(root);

        while (!stack1.isEmpty()) {

            Node current = stack1.pop();
            stack2.push(current);

            if (current.left != null)
                stack1.push(current.left);

            if (current.right != null)
                stack1.push(current.right);
        }

        while (!stack2.isEmpty()) {
            System.out.print(stack2.pop().data + " ");
        }
    }

    public static void main(String[] args) {

        PostorderIterative tree = new PostorderIterative();

        tree.root = new Node(10);
        tree.root.left = new Node(20);
        tree.root.right = new Node(30);
        tree.root.left.left = new Node(40);
        tree.root.left.right = new Node(50);

        System.out.print("Postorder Traversal: ");
        tree.postorder();
    }
}

Output:

Postorder Traversal: 40 50 20 30 10

Time and Space Complexity: Time Complexity: O(n)
Space Complexity: O(h)
where, h = height of the binary tree

Balanced Tree → O(log n)
Skewed Tree → O(n)

To wrapping up with

Postorder Traversal in Binary Tree in Java:

If root is null → traversal returns nothing
Check null before accessing child nodes
Ensure stacks are not empty before pop

Common Problem with Postorder traversal:

Forgetting correct order (Left → Right → Root)
Mixing preorder logic in iterative solution
Incorrect stack usage
Missing base condition

Frequently Asked Questions

1. What is postorder traversal in binary tree in Java?

Answer:

It is a traversal method where nodes are visited in Left → Right → Root order.

2. What is the time complexity of postorder traversal?

Answer:

Time complexity is O(n) because all nodes are visited once.

3. Why is postorder traversal used?

Answer:

It is used in tree deletion and postfix expression evaluation.

4. Can postorder traversal be done iteratively?

Answer:

Yes, using one or two stacks.

5. What is the space complexity of postorder traversal?

Answer:

Space complexity is O(h), where h is the height of the tree.In

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Postorder Traversal in Binary Tree in java