Move vowels node at the beginning of linked list in scala

Scala program for Move vowels node at the beginning of linked list. Here more information.

/*
  Scala program for
  Shift the vowels node at beginning of linked list
*/
// Linked list node
class LinkNode(var data: Char,
	var next: LinkNode)
{
	def this(data: Char)
	{
		this(data, null)
	}
}
class SingleLL(var head: LinkNode,
	var tail: LinkNode)
{
	def this()
	{
		this(null, null)
	}
	// Add new node at end of linked list 
	def addNode(data: Char): Unit = {
		var node: LinkNode = new LinkNode(data);
		if (this.head == null)
		{
			this.head = node;
		}
		else
		{
			// Append the node at last position
			this.tail.next = node;
		}
		this.tail = node;
	}
	// Display linked list element
	def display(): Unit = {
		if (this.head == null)
		{
			println("\n Empty linked list");
			return;
		}
		var temp: LinkNode = this.head;
		// iterating linked list elements
		while (temp != null)
		{
			print(" " + temp.data + " →");
			// Visit to next node
			temp = temp.next;
		}
		print(" NULL\n");
	}
	// Determine that given value is an vowel or not
	def isVowels(value: Char): Boolean = {
		if (value == 'a' || value == 'e' || 
             value == 'i' || value == 'o' || 
             value == 'u' || value == 'A' || 
             value == 'E' || value == 'I' || 
             value == 'O' || value == 'U')
		{
			return true;
		}
		return false;
	}
	// Move the vowels value at beginning position
	def shiftVowels(): Unit = {
		if (this.head == null)
		{
			print("\n Empty linked list\n");
			return;
		}
		// Define some auxiliary variable
		var node: LinkNode = this.head;
		var back: LinkNode = null;
		// iterate linked list nodes
		// And shift vowels at front of linked list
		while (node != null)
		{
			if (isVowels(node.data) == true && back != null)
			{
				// Enter here when
				// Node is contain vowel and 
				// its previous (any) node is not form of vowel
				if (node == this.tail)
				{
					// When is last node
					// Set new last node
					this.tail = back;
				}
				// Connect previous node to next node
				back.next = node.next;
				// Connect vowel node to first node
				node.next = this.head;
				// Set new head
				this.head = node;
				// Get back node
				node = back;
			}
			else
			{
				back = node;
			}
			// Visit to next node
			node = node.next;
		}
	}
}
object Main
{
	def main(args: Array[String]): Unit = {
		var sll: SingleLL = new SingleLL();
		// Constructed linked list
		sll.addNode('E');
		sll.addNode('D');
		sll.addNode('U');
		sll.addNode('C');
		sll.addNode('A');
		sll.addNode('T');
		sll.addNode('I');
		sll.addNode('O');
		sll.addNode('N');
		println(" Before move vowels");
		// E → D → U → C → A → T → I → O → N → NULL
		sll.display();
		sll.shiftVowels();
		println(" After move vowels");
		// O → I → A → U → E → D → C → T → N → NULL
		sll.display();
	}
}

Output

 Before move vowels
 E → D → U → C → A → T → I → O → N → NULL
 After move vowels
 O → I → A → U → E → D → C → T → N → NULL