Flatten binary tree in order of postorder traversal in c++

C++ program for Flatten binary tree in order of postorder traversal. Here more solutions.

// Include header file
#include <iostream>
using namespace std;
/*
  C++ program for
  Flatten binary tree in order of post-order traversal
*/
// Node of Binary Tree
class TreeNode
{
	public: 
    int data;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int data)
	{
		// Set node value
		this->data = data;
		this->left = nullptr;
		this->right = nullptr;
	}
};
class BinaryTree
{
	public: 
    TreeNode *root;
	TreeNode *back;
	BinaryTree()
	{
		this->root = nullptr;
		this->back = nullptr;
	}
	// Recursive function
	// Display postorder view of binary tree
	void postOrder(TreeNode *node)
	{
		if (node != nullptr)
		{
			this->postOrder(node->left);
			this->postOrder(node->right);
			// Print node value
			cout << "  " << node->data;
		}
	}
	// This are flattening tree nodes in postorder from
	void transform(TreeNode *node)
	{
		if (node != nullptr)
		{
			// Recursive executing left and right subtree
			this->transform(node->left);
			this->transform(node->right);
			if (this->back == nullptr)
			{
				// 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 = nullptr;
			}
			this->back = node;
		}
	}
	// This are handling the request of
	// flatten tree nodes in post order from.
	void flattenNode()
	{
		if (this->root == nullptr)
		{
			// When empty tree
			return;
		}
		// Set back node
		this->back = nullptr;
		// Perform flatten operation
		this->transform(this->root);
		if (this->back != nullptr)
		{
			// Set last node of post order
			this->back->left = nullptr;
			this->back->right = nullptr;
		}
		this->back = nullptr;
	}
	// Display flatten elements of tree
	void showElement()
	{
		if (this->root == nullptr)
		{
			cout << "\n Empty Tree" << endl;
			return;
		}
		cout << "\n Flatten Tree Node in Postorder : " << endl;
		TreeNode *temp = this->root;
		// Iterate tree elements
		while (temp != nullptr)
		{
			// Display node value
			cout << "  " << temp->data;
			// Visit to next node
			temp = temp->right;
		}
	}
};
int main()
{
	// 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
	cout << "\n Postorder Nodes : " << endl;
	tree->postOrder(tree->root);
	// Testing
	tree->flattenNode();
	// After transform
	tree->showElement();
	return 0;
}

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