Question : Binary Tree PostOrder traversal in java

 PostOrder traversal

In PostOrder traversal, each node is processed after subtrees traversal.In simpler words,Visit left subtree,  right subtree and then node.

Steps for PostOrder traversal are:

• Traverse the left subtree in PostOrder.
• Traverse the right subtree in PostOrder.
• Visit the node

There can be two ways of implementing it

• Recursive
• Iterative

Recursive solution

The recursive solution is very straightforward. The below diagram will make you understand recursion better.

Code for recursion will be:

// Recursive Solution
    public void postOrder(TreeNode root) {
        if(root !=  null) {
            postOrder(root.left);
            postOrder(root.right);
            //Visit the node by Printing the node data  
            System.out.printf("%d ",root.data);
        }
    }

Iterative solution:

Steps for iterative solution:

1. Create an empty stack s and set currentNode =root.
2. while currentNode is not NULL Do following
   1. Push currentNode 's right child and then currentNode to stack s
   2. Set currentNode=currentNode.left
3. Pop a node from stack s and set it to currentNode
   1. If the popped node has a right child and the right child is at top of stack, then remove the right child from stack, push the currentNode back and set currentNode as currentNode 's right child.
   2. Else print currentNode's data and set currentNode as NULL.
4. Repeat steps 2 and 3 while stack is not empty.

// Iterative solution
    public void postorderIter( TreeNode root) {
        if( root == null ) return;
 
        Stack<TreeNode> s = new Stack<TreeNode>( );
        TreeNode current = root;
 
        while( true ) {
 
            if( current != null ) {
                if( current.right != null )
                    s.push( current.right );
                s.push( current );
                current = current.left;
                continue;
            }
 
            if( s.isEmpty( ) )
                return;
            current = s.pop( );
 
            if( current.right != null && ! s.isEmpty( ) && current.right == s.peek( ) ) {
                s.pop( );
                s.push( current );
                current = current.right;
            } else {
                System.out.print( current.data + " " );
                current = null;
            }
        }
    }

Let's create a java program for postOrder traversal:

 
import java.util.Stack;
 
public class BinaryTreePostOrder {
 
    public static class TreeNode
    {
        int data;
        TreeNode left;
        TreeNode right;
        TreeNode(int data)
        {
            this.data=data;
        }
    }
 
    // Recursive Solution
    public void postOrder(TreeNode root) {
        if(root !=  null) {
            postOrder(root.left);
            postOrder(root.right);
            //Visit the node by Printing the node data  
            System.out.printf("%d ",root.data);
        }
    }
 
    // Iterative solution
    public void postorderIter( TreeNode root) {
        if( root == null ) return;
 
        Stack<TreeNode> s = new Stack<TreeNode>( );
        TreeNode current = root;
 
        while( true ) {
 
            if( current != null ) {
                if( current.right != null )
                    s.push( current.right );
                s.push( current );
                current = current.left;
                continue;
            }
 
            if( s.isEmpty( ) )
                return;
            current = s.pop( );
 
            if( current.right != null && ! s.isEmpty( ) && current.right == s.peek( ) ) {
                s.pop( );
                s.push( current );
                current = current.right;
            } else {
                System.out.print( current.data + " " );
                current = null;
            }
        }
    }
 
    public static void main(String[] args)
    {
        BinaryTreePostOrder bi=new BinaryTreePostOrder();
        // Creating a binary tree
        TreeNode rootNode=createBinaryTree();
        System.out.println("Using Recursive solution:");
 
        bi.postOrder(rootNode);
 
        System.out.println();
        System.out.println("-------------------------");
        System.out.println("Using Iterative solution:");
 
        bi.postorderIter(rootNode);
    }
 
    public static TreeNode createBinaryTree()
    {
 
        TreeNode rootNode =new TreeNode(40);
        TreeNode node20=new TreeNode(20);
        TreeNode node10=new TreeNode(10);
        TreeNode node30=new TreeNode(30);
        TreeNode node60=new TreeNode(60);
        TreeNode node50=new TreeNode(50);
        TreeNode node70=new TreeNode(70);
 
        rootNode.left=node20;
        rootNode.right=node60;
 
        node20.left=node10;
        node20.right=node30;
 
        node60.left=node50;
        node60.right=node70;
 
        return rootNode;
    }
}

Run above program and you will get following output:

Using Recursive solution:
10 30 20 50 70 60 40
————————-
Using Iterative solution:
10 30 20 50 70 60 40