Arrange consonants and vowels nodes in a linked list in swift

Swift program for Arrange consonants and vowels nodes in a linked list. Here more information.

import Foundation
/*
  Swift 4 program for 
  Arrange consonants and vowels nodes in a 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 appendNode(_ node: LinkNode? )
	{
		if (self.head == nil)
		{
			self.head = node;
		}
		else
		{
			// Append the node at last position
			self.tail!.next = node;
		}
		self.tail = node;
	}
	// Handles the request of adding new node in linked list
	func addNode(_ data: Character)
	{
		let node: LinkNode? = LinkNode(data);
		self.appendNode(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\n", terminator: "");
	}
	// 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;
	}
	// Arrange the nodes in front position of vowels 
	// and end with consonants
	func arrangeNodes()
	{
		if (self.head == nil)
		{
			print("\n Empty linked list");
			return;
		}
		// Define some auxiliary variable
		let vowel: SingleLL? = SingleLL();
		let consonant: SingleLL? = SingleLL();
		var auxiliary: LinkNode? = self.head;
		var ordinary: LinkNode? = nil;
		// iterate linked list nodes
		while (auxiliary  != nil)
		{
			ordinary = auxiliary;
			auxiliary = auxiliary!.next;
			if (self.isVowels(ordinary!.data) == true)
			{
				// Add node into volwel list
				vowel!.appendNode(ordinary);
			}
			else
			{
				consonant!.appendNode(ordinary);
			}
			ordinary!.next = nil;
		}
		if (vowel!.head  != nil)
		{
			// Set new head node of resultant linked list
			self.head = vowel!.head;
			// Connect vowel and consonant linked list
			vowel!.tail!.next = consonant!.head;
			if (consonant!.tail  != nil)
			{
				// When consonant node exists
				// Set new tail node of resultant linked list 
				self.tail = consonant!.tail;
			}
			else
			{
				// When consonant nodes not exists
				self.tail = vowel!.tail;
			}
		}
		else
		{
			// When vowels not exist
			self.head = consonant!.head;
			self.tail = consonant!.tail;
		}
	}
	static func main()
	{
		let sll: SingleLL = SingleLL();
		// Constructed linked list
		sll.addNode("R");
		sll.addNode("E");
		sll.addNode("G");
		sll.addNode("U");
		sll.addNode("L");
		sll.addNode("A");
		sll.addNode("T");
		sll.addNode("I");
		sll.addNode("O");
		sll.addNode("N");
		print(" Before Arrange");
		// R → E → G → U → L → A → T → I → O → N → NULL
		sll.display();
		sll.arrangeNodes();
		print(" After Arrange");
		// E → U → A → I → O → R → G → L → T → N → NULL
		sll.display();
	}
}
SingleLL.main();

Output

 Before Arrange
 R → E → G → U → L → A → T → I → O → N → NULL
 After Arrange
 E → U → A → I → O → R → G → L → T → N → NULL