Count frequency of a key in doubly linked list in scala

Scala program for Count frequency of a key in doubly linked list. Here more information.

// Scala program for
// Count frequency of given node in doubly linked list
class LinkNode(var data: Int,
	var next: LinkNode,
		var prev: LinkNode)
{
	def this(data: Int)
	{
		this(data, null, null);
	}
}
class DoublyLinkedList(var head: LinkNode,
	var tail: LinkNode)
{
	def this()
	{
		this(null, null);
	}
	// Insert new node at end position
	def insert(value: Int): Unit = {
		// Create a node
		var node: LinkNode = new LinkNode(value);
		if (this.head == null)
		{
			// 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
	def display(): Unit = {
		if (this.head == null)
		{
			println("Empty Linked List");
		}
		else
		{
			print("Linked List Head to Tail :");
			// Get first node of linked list
			var temp: LinkNode = this.head;
			// iterate linked list 
			while (temp != null)
			{
				// Display node value
				print("  " + temp.data);
				// Visit to next node
				temp = temp.next;
			}
			print("\nLinked List Tail to Head :");
			// Get last node of linked list
			temp = this.tail;
			// iterate linked list 
			while (temp != null)
			{
				// Display node value
				print("  " + temp.data);
				// Visit to prev node
				temp = temp.prev;
			}
			print("\n");
		}
	}
	// Count frequency of given node
	def frequency(key: Int): Unit = {
		var result: Int = 0;
		var temp: LinkNode = this.head;
		while (temp != null)
		{
			if (temp.data == key)
			{
				// count key
				result += 1;
			}
			// Visit to next node
			temp = temp.next;
		}
		// Print result
		println("Frequency of node " + key + " is : " + result);
	}
}
object Main
{
	def main(args: Array[String]): Unit = {
		var dll: DoublyLinkedList = 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);
	}
}

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