Count frequency of a key in doubly linked list in c++

C++ program for Count frequency of a key in doubly linked list. Here problem description and other solutions.

// Include header file
#include <iostream>
using namespace std;
// C++ program for
// Count frequency of given node in doubly linked list
class LinkNode
{
	public: int data;
	LinkNode *next;
	LinkNode *prev;
	LinkNode(int data)
	{
		this->data = data;
		this->next = nullptr;
		this->prev = nullptr;
	}
};
class DoublyLinkedList
{
	public: LinkNode *head;
	LinkNode *tail;
	DoublyLinkedList()
	{
		this->head = nullptr;
		this->tail = nullptr;
	}
	// Insert new node at end position
	void insert(int value)
	{
		// Create a node
		LinkNode *node = new LinkNode(value);
		if (this->head == nullptr)
		{
			// Add first node
			this->head = node;
			this->tail = node;
			return;
		}
		// Add node at last position
		this->tail->next = node;
		node->prev = this->tail;
		this->tail = node;
	}
	// Display node element of doubly linked list
	void display()
	{
		if (this->head == nullptr)
		{
			cout << "Empty Linked List" << endl;
		}
		else
		{
			cout << "Linked List Head to Tail :";
			// Get first node of linked list
			LinkNode *temp = this->head;
			// iterate linked list 
			while (temp != nullptr)
			{
				// Display node value
				cout << "  " << temp->data;
				// Visit to next node
				temp = temp->next;
			}
			cout << "\nLinked List Tail to Head :";
			// Get last node of linked list
			temp = this->tail;
			// iterate linked list 
			while (temp != nullptr)
			{
				// Display node value
				cout << "  " << temp->data;
				// Visit to prev node
				temp = temp->prev;
			}
			cout << "\n";
		}
	}
	// Count frequency of given node
	void frequency(int key)
	{
		int result = 0;
		LinkNode *temp = this->head;
		while (temp != nullptr)
		{
			if (temp->data == key)
			{
				// count key
				result++;
			}
			// Visit to next node
			temp = temp->next;
		}
		// Print result
		cout << "Frequency of node " << key 
             << " is : " << result << endl;
	}
};
int main()
{
	DoublyLinkedList *dll = new DoublyLinkedList();
	// Insert following linked list nodes
	dll->insert(7);
	dll->insert(2);
	dll->insert(3);
	dll->insert(4);
	dll->insert(5);
	dll->insert(3);
	dll->insert(7);
	dll->insert(3);
	//display all node
	dll->display();
	dll->frequency(3);
	dll->frequency(11);
	dll->frequency(7);
	return 0;
}

Output

Linked List Head to Tail :  7  2  3  4  5  3  7  3
Linked List Tail to Head :  3  7  3  5  4  3  2  7
Frequency of node 3 is : 3
Frequency of node 11 is : 0
Frequency of node 7 is : 2