Insertion sort on doubly linked list in swift

Swift program for Insertion sort on doubly linked list. Here problem description and other solutions.

import Foundation
// Swift 4 program for
// Perform insertion sort on doubly linked list
class LinkNode
{
	var data: Int;
	var next: LinkNode? ;
	var prev: LinkNode? ;
	init(_ data: Int)
	{
		self.data = data;
		self.next = nil;
		self.prev = nil;
	}
}
class DoublyLinkedList
{
	var head: LinkNode? ;
	var tail: LinkNode? ;
	init()
	{
		self.head = nil;
		self.tail = nil;
	}
	// Insert new node at end position
	func insert(_ value: Int)
	{
		// Create a node
		let node: LinkNode? = LinkNode(value);
		if (self.head == nil)
		{
			// Add first node
			self.head = node;
			self.tail = node;
			return;
		}
		// Add node at last position
		self.tail!.next = node;
		node!.prev = self.tail;
		self.tail = node;
	}
	// Display node element of doubly linked list
	func display()
	{
		if (self.head == nil)
		{
			print("Empty Linked List");
		}
		else
		{
			print("\nLinked List Head to Tail ", terminator: ":");
			// Get first node of linked list
			var temp: LinkNode? = self.head;
			// iterate linked list 
			while (temp  != nil)
			{
				// Display node value
				print("",temp!.data, terminator: " ");
				// Visit to next node
				temp = temp!.next;
			}
			print("\nLinked List Tail to Head ", terminator: ":");
			// Get last node of linked list
			temp = self.tail;
			// iterate linked list 
			while (temp  != nil)
			{
				// Display node value
				print("",temp!.data, terminator: " ");
				// Visit to prev node
				temp = temp!.prev;
			}
			print();
		}
	}
	// Swap value of given node
	func swapData(_ first: LinkNode? , _ second : LinkNode? )
	{
		let value: Int = first!.data;
		first!.data = second!.data;
		second!.data = value;
	}
	// Sort elements using insertion sort
	func insertionSort()
	{
		// Get first node
		var front: LinkNode? = self.head;
		var back: LinkNode? = nil;
		while (front  != nil)
		{
			// Get next node
			back = front!.next;
			// Update node value when consecutive nodes are not sort
			while (back  != nil && back!.prev  != nil && back!.data < back!.prev!.data)
			{
				// Modified node data
				self.swapData(back, back!.prev);
				// Visit to previous node
				back = back!.prev;
			}
			// Visit to next node
			front = front!.next;
		}
	}
	static func main(_ args: [String])
	{
		let dll: DoublyLinkedList? = DoublyLinkedList();
		//Insert element of linked list
		dll!.insert(25);
		dll!.insert(2);
		dll!.insert(6);
		dll!.insert(14);
		dll!.insert(12);
		dll!.insert(9);
		dll!.insert(16);
		dll!.insert(3);
		print("\n Before Sort", terminator: "");
		dll!.display();
		// Display all node
		dll!.insertionSort();
		print("\n After Sort", terminator: "");
		dll!.display();
	}
}
DoublyLinkedList.main([String]());

Output

 Before Sort
Linked List Head to Tail : 25  2  6  14  12  9  16  3
Linked List Tail to Head : 3  16  9  12  14  6  2  25

 After Sort
Linked List Head to Tail : 2  3  6  9  12  14  16  25
Linked List Tail to Head : 25  16  14  12  9  6  3  2