Fold linked list in swift

Swift program for Fold linked list. Here more information.

import Foundation
/*
  Swift 4 program for
  Fold 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? ;
	init()
	{
		self.head = nil;
	}
	// Add node at the beginning
	func addNode(_ data: Int)
	{
		// Create new node
		let node: LinkNode? = LinkNode(data);
		node!.next = self.head;
		// Set new head
		self.head = 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");
	}
	// This is handle the request of 
	// folding given linked list nodes
	func foldList()
	{
		if (self.head == nil)
		{
			print("\n Empty linked list");
			return;
		}
		// Some auxiliary variables
		var temp: LinkNode? = self.head;
		var auxiliary: LinkNode? = self.head;
		var hold: LinkNode? = nil;
		// Loops used to find the middle node
		while (temp  != nil && 
          temp!.next  != nil && 
          temp!.next!.next  != nil)
		{
			// Visit to next node
			auxiliary = auxiliary!.next;
			// Visit to second next node
			temp = temp!.next!.next;
		}
		if (auxiliary  != nil)
		{
			hold = auxiliary;
			auxiliary = auxiliary!.next;
			// Separating half nodes
			hold!.next = nil;
			// Start with first node of next half section
			temp = auxiliary;
			auxiliary = nil;
			// Reverse second half nodes 
			while (temp  != nil)
			{
				hold = temp;
				temp = temp!.next;
				hold!.next = auxiliary;
				auxiliary = hold;
			}
			// Start with first node of linked list
			temp = self.head;
			// Combine first and second half nodes 
			// in alternate manner
			while (temp  != nil)
			{
				hold = temp;
				temp = temp!.next;
				hold!.next = auxiliary;
				if (auxiliary  != nil)
				{
					hold = auxiliary;
					auxiliary = auxiliary!.next;
					hold!.next = temp;
				}
			}
		}
	}
	static func main()
	{
		// Create a empty linked lists
		let sll1: SingleLL = SingleLL();
		let sll2: SingleLL = SingleLL();
		//  Constructed first linked list
		//  1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 → NULL
		sll1.addNode(8);
		sll1.addNode(7);
		sll1.addNode(6);
		sll1.addNode(5);
		sll1.addNode(4);
		sll1.addNode(3);
		sll1.addNode(2);
		sll1.addNode(1);
		// Constructed second linked list
		//  1 → 2 → 3 → 4 → 5 → 6 → 7 → NULL
		sll2.addNode(7);
		sll2.addNode(6);
		sll2.addNode(5);
		sll2.addNode(4);
		sll2.addNode(3);
		sll2.addNode(2);
		sll2.addNode(1);
		// Test A
		print(" Before folding");
		sll1.display();
		// Form of folding
		//      1 → 2 → 3 → 4 ──┐
		// NULL ← 8 ← 7 ← 6 ← 5⤶
		sll1.foldList();
		// After merging
		// 1 → 8 → 2 → 7 → 3 → 6 → 4 → 5 → NULL
		print(" After folding");
		sll1.display();
		// Test B
		print(" Before folding");
		sll2.display();
		// Form of folding
		//      1 → 2 → 3 → 4 
		// NULL ← 7 ← 6 ← 5⤶
		sll2.foldList();
		// After merging
		// 1 → 7 → 2 → 6 → 3 → 5 → 4 → NULL
		print(" After folding");
		sll2.display();
	}
}
SingleLL.main();

Output

 Before folding
 1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 → NULL
 After folding
 1 → 8 → 2 → 7 → 3 → 6 → 4 → 5 → NULL
 Before folding
 1 → 2 → 3 → 4 → 5 → 6 → 7 → NULL
 After folding
 1 → 7 → 2 → 6 → 3 → 5 → 4 → NULL