Flatten binary tree in order of postorder traversal in java

Java program for Flatten binary tree in order of postorder traversal. Here mentioned other language solution.

/* 
  Java program for
  Flatten binary tree in order of post-order traversal
*/

// Node of Binary Tree
class TreeNode
{
    public int data;
    public TreeNode left;
    public TreeNode right;
    public TreeNode(int data)
    {
        // Set node value
        this.data = data;
        this.left = null;
        this.right = null;
    }
}
class BinaryTree
{
    public TreeNode root;
    public TreeNode back;
    public BinaryTree()
    {
        // Set initial tree root to null
        this.root = null;
        this.back = null;
    }
    // Recursive function
    // Display postorder view of binary tree
    public void postOrder(TreeNode node)
    {
        if (node != null)
        {
            postOrder(node.left);
            postOrder(node.right);
            // Print node value
            System.out.print("  " + node.data);
        }
    }
    // This are flattening tree nodes in postorder from
    public void transform(TreeNode node)
    {
        if (node != null)
        {
            // Recursive executing left and right subtree
            transform(node.left);
            transform(node.right);
            if (this.back == null)
            {
                // This is first node of postorder traversal
                // Get first node of transform tree
                this.root = node;
            }
            else
            {
                // Next node
                this.back.right = node;
                // We taking only one direction
                this.back.left = null;
            }
            this.back = node;
        }
    }
    // This are handling the request of 
    // flatten tree nodes in post order from.
    public void flattenNode()
    {
        if (this.root == null)
        {
            // When empty tree
            return;
        }
        // Set back node
        this.back = null;
        // Perform flatten operation
        transform(this.root);
        if (this.back != null)
        {
            // Set last node of post order
            this.back.left = null;
            this.back.right = null;
        }
        this.back = null;
    }
    // Display flatten elements of tree
    public void showElement()
    {
        if (this.root == null)
        {
            System.out.println("\n Empty Tree");
            return;
        }
        System.out.println("\n Flatten Tree Node in Postorder : ");
        TreeNode temp = this.root;
        // Iterate tree elements
        while (temp != null)
        {
            // Display node value
            System.out.print("  " + temp.data);
            // Visit to next node
            temp = temp.right;
        }
    }
    public static void main(String[] args)
    {
        // New tree
        BinaryTree tree = new BinaryTree();
        /* 
            Construct Binary Tree
            -----------------------
                   1
                  / \ 
                 /   \
                6     8
               / \   / \
              2   3 7   5
             /   /   \   \
            9   4    -6   11
        */
        // Add nodes
        tree.root = new TreeNode(1);
        tree.root.left = new TreeNode(6);
        tree.root.left.left = new TreeNode(2);
        tree.root.right = new TreeNode(8);
        tree.root.right.right = new TreeNode(5);
        tree.root.right.left = new TreeNode(7);
        tree.root.right.left.right = new TreeNode(-6);
        tree.root.left.right = new TreeNode(3);
        tree.root.left.right.left = new TreeNode(4);
        tree.root.left.left.left = new TreeNode(9);
        tree.root.right.right.right = new TreeNode(11);
        // Display tree elements
        System.out.println("\n Postorder Nodes : ");
        tree.postOrder(tree.root);
        // Testing
        tree.flattenNode();
        // After transform
        tree.showElement();
    }
}

Output

 Postorder Nodes :
  9  2  4  3  6  -6  7  11  5  8  1
 Flatten Tree Node in Postorder :
  9  2  4  3  6  -6  7  11  5  8  1