Move vowels node at the beginning of linked list in swift

Swift program for Move vowels node at the beginning of linked list. Here problem description and other solutions.

import Foundation
/*
  Swift 4 program for
  Shift the vowels node at beginning of linked list
*/
// Linked list node
class LinkNode
{
	var data: Character;
	var next: LinkNode? ;
	init(_ data: Character)
	{
		self.data = data;
		self.next = nil;
	}
}
class SingleLL
{
	var head: LinkNode? ;
	var tail: LinkNode? ;
	init()
	{
		self.head = nil;
		self.tail = nil;
	}
	// Add new node at end of linked list 
	func addNode(_ data: Character)
	{
		let node: LinkNode? = LinkNode(data);
		if (self.head == nil)
		{
			self.head = node;
		}
		else
		{
			// Append the node at last position
			self.tail!.next = node;
		}
		self.tail = node;
	}
	// Display linked list element
	func display()
	{
		if (self.head == nil)
		{
			print("\n Empty linked list");
			return;
		}
		var temp: LinkNode? = self.head;
		// iterating linked list elements
		while (temp  != nil)
		{
			print("",temp!.data, terminator: " →");
			// Visit to next node
			temp = temp!.next;
		}
		print(" NULL");
	}
	// Determine that given value is an vowel or not
	func isVowels(_ value: Character) -> Bool
	{
		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
	func shiftVowels()
	{
		if (self.head == nil)
		{
			print("\n Empty linked list");
			return;
		}
		// Define some auxiliary variable
		var node: LinkNode? = self.head;
		var back: LinkNode? = nil;
		// iterate linked list nodes
		// And shift vowels at front of linked list
		while (node  != nil)
		{
			if (self.isVowels(node!.data) == true && back  != nil)
			{
				// Enter here when
				// Node is contain vowel and 
				// its previous (any) node is not form of vowel
				if (node === self.tail)
				{
					// When is last node
					// Set new last node
					self.tail = back;
				}
				// Connect previous node to next node
				back!.next = node!.next;
				// Connect vowel node to first node
				node!.next = self.head;
				// Set new head
				self.head = node;
				// Get back node
				node = back;
			}
			else
			{
				back = node;
			}
			// Visit to next node
			node = node!.next;
		}
	}
	static func main()
	{
		let sll: SingleLL = 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");
		print(" Before move vowels");
		// E → D → U → C → A → T → I → O → N → NULL
		sll.display();
		sll.shiftVowels();
		print(" After move vowels");
		// O → I → A → U → E → D → C → T → N → NULL
		sll.display();
	}
}
SingleLL.main();

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