Subtraction of alternate nodes of linked list in swift

Swift program for Subtraction of alternate nodes of linked list. Here problem description and other solutions.

import Foundation
/*
    Swift 4 program for
    Subtraction of the alternate nodes of linked list
*/
// Linked list node
class LinkNode
{
	var data: Int;
	var next: LinkNode? ;
	init(_ data: Int)
	{
		self.data = data;
		self.next = nil;
	}
}
class SingleLL
{
	var head: LinkNode? ;
	var tail: LinkNode? ;
	init()
	{
		self.head = nil;
		self.tail = nil;
	}
	func insert(_ data: Int)
	{
		let node: LinkNode? = LinkNode(data);
		if (self.head == nil)
		{
			// Add first node
			self.head = node;
		}
		else
		{
			// Add node at the end position
			self.tail!.next = node;
		}
		// New last node
		self.tail = node;
	}
	// Display linked list element
	func display()
	{
		if (self.head == nil)
		{
			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");
	}
	// Find the subtraction of all alternate 
	// nodes in linked list
	func alternateSubtraction()
	{
		// Define resultant variables
		var result: Int = 0;
		var counter: Int = 0;
		// Start to first node of linked list
		var temp: LinkNode? = self.head;
		// iterating linked list elements
		while (temp  != nil)
		{
			if (counter % 2 == 0)
			{
				// When get alternate node
				if (result == 0)
				{
					result = temp!.data;
				}
				else
				{
					result = result - temp!.data;
				}
			}
			// Node counter
			counter += 1;
			// Visit to next node
			temp = temp!.next;
		}
		print(" Alternate nodes subtraction :",result);
	}
	static func main()
	{
		let sll: SingleLL = SingleLL();
		// Add node in linked list
		// 4 → 7 → 2 → 9 → 1 → 3 → 4 → 3 → 6 → NULL
		sll.insert(4);
		sll.insert(7);
		sll.insert(2);
		sll.insert(9);
		sll.insert(1);
		sll.insert(3);
		sll.insert(4);
		sll.insert(3);
		sll.insert(6);
		// Display of linked list nodes
		sll.display();
		// Test
		sll.alternateSubtraction();
	}
}
SingleLL.main();

Output

  4 →  7 →  2 →  9 →  1 →  3 →  4 →  3 →  6 → NULL
 Alternate nodes subtraction : -9