Insertion in binary search tree using recursion in c++

C++ program for Insertion in binary search tree using recursion. Here more information.

// Include header file
#include <iostream>

using namespace std;
// C++ program for
// Insertion in binary search tree by using recursion
class TreeNode
{
	public: int data;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int data)
	{
		this->data = data;
		this->left = nullptr;
		this->right = nullptr;
	}
};
class BinarySearchTree
{
	public: TreeNode *root;
	BinarySearchTree()
	{
		this->root = nullptr;
	}
	// Insert a node element
	TreeNode *addNode(TreeNode *node, int data)
	{
		if (node != nullptr)
		{
			if (node->data >= data)
			{
				// When new element is smaller or
				// equal to current node
				node->left = this->addNode(node->left, data);
			}
			else
			{
				// When new element is higher to current node
				node->right = this->addNode(node->right, data);
			}
			// important to manage root node
			return node;
		}
		else
		{
			return new TreeNode(data);
		}
	}
	// Display preorder
	void preorder(TreeNode *node)
	{
		if (node != nullptr)
		{
			// Display node value
			cout << "  " << node->data;
			// Visit to left subtree
			this->preorder(node->left);
			// Visit to right subtree
			this->preorder(node->right);
		}
	}
	void inorder(TreeNode *node)
	{
		if (node != nullptr)
		{
			// Visit to left subtree
			this->inorder(node->left);
			// Display node value
			cout << "  " << node->data;
			// Visit to right subtree
			this->inorder(node->right);
		}
	}
	void postorder(TreeNode *node)
	{
		if (node != nullptr)
		{
			// Visit to left subtree
			this->postorder(node->left);
			// Visit to right subtree
			this->postorder(node->right);
			// Display node value
			cout << "  " << node->data;
		}
	}
};
int main()
{
	BinarySearchTree *tree = new BinarySearchTree();
	/*
	         10
	        / \
	       /   \
	      4     15
	     / \   /
	    3   5 12
	    -------------
	    Build binary search tree
	*/
	tree->root = tree->addNode(tree->root, 10);
	tree->addNode(tree->root, 4);
	tree->addNode(tree->root, 3);
	tree->addNode(tree->root, 5);
	tree->addNode(tree->root, 15);
	tree->addNode(tree->root, 12);
	// Display tree nodes
	cout << "Preorder " << endl;
	tree->preorder(tree->root);
	cout << "\nInorder " << endl;
	tree->inorder(tree->root);
	cout << "\nPostorder " << endl;
	tree->postorder(tree->root);
	return 0;
}

Output

Preorder
  10  4  3  5  15  12
Inorder
  3  4  5  10  12  15
Postorder
  3  5  4  12  15  10