Euler tour of binary tree in c++

C++ program for Euler tour of binary tree. Here more information.

// Include header file
#include <iostream>
using namespace std;
/*
  C++ program for
  Euler tour of a binary tree
*/
// Binary Tree Node
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;
	BinaryTree()
	{
		this->root = nullptr;
	}
	// Recursively Printing the elements of euler path
	void eulerPath(TreeNode *node, TreeNode *parent)
	{
		if (node == nullptr)
		{
			// When tree node empty
			return;
		}
		// Display node value
		cout << "  " << node->data;
		// Recursively visit the left and right subtree
		this->eulerPath(node->left, node);
		this->eulerPath(node->right, node);
		if (parent != nullptr)
		{
			// When parent not null
			cout << "   " << parent->data;
		}
	}
};
int main()
{
	// Create new tree
	BinaryTree *tree = new BinaryTree();
	/*
	        9
	       / \
	      5   7
	     / \   \
	    1   6   11
	   /   / \   \
	  10  4   8   20
	         / \
	        2   3
	  -----------------------
	    Binary Tree
	*/
	tree->root = new TreeNode(9);
	tree->root->left = new TreeNode(5);
	tree->root->right = new TreeNode(7);
	tree->root->left->left = new TreeNode(1);
	tree->root->left->left->left = new TreeNode(10);
	tree->root->left->right = new TreeNode(6);
	tree->root->left->right->left = new TreeNode(4);
	tree->root->left->right->right = new TreeNode(8);
	tree->root->left->right->right->left = new TreeNode(2);
	tree->root->left->right->right->right = new TreeNode(3);
	tree->root->right->right = new TreeNode(11);
	tree->root->right->right->right = new TreeNode(20);
	tree->eulerPath(tree->root, nullptr);
	return 0;
}

Output

  9  5  1  10   1   5  6  4   6  8  2   8  3   8   6   5   9  7  11  20   11   7   9