Fold linked list in kotlin

Kotlin program for Fold linked list. Here problem description and explanation.

/*
  Kotlin program for
  Fold linked list
*/
// Linked list node
class LinkNode
{
	var data: Int;
	var next: LinkNode ? ;
	constructor(data: Int)
	{
		this.data = data;
		this.next = null;
	}
}
class SingleLL
{
	var head: LinkNode ? ;
	constructor()
	{
		this.head = null;
	}
	// Add node at the beginning
	fun addNode(data: Int): Unit
	{
		// Create new node
		val node: LinkNode = LinkNode(data);
		node.next = this.head;
		// Set new head
		this.head = node;
	}
	// Display linked list element
	fun display(): Unit
	{
		if (this.head == null)
		{
			return;
		}
		var temp: LinkNode ? = this.head;
		// iterating linked list elements
		while (temp != null)
		{
			print(" " + temp.data + " →");
			// Visit to next node
			temp = temp.next;
		}
		println(" NULL");
	}
	// This is handle the request of 
	// folding given linked list nodes
	fun foldList(): Unit
	{
		if (this.head == null)
		{
			print("\n Empty linked list");
			return;
		}
		// Some auxiliary variables
		var temp: LinkNode ? = this.head;
		var auxiliary: LinkNode ? = this.head;
		var hold: LinkNode ? ;
		// Loops used to find the middle node
		while (temp != null && temp.next != null && 
          temp.next?.next != null)
		{
			// Visit to next node
			auxiliary = auxiliary?.next;
			// Visit to second next node
			temp = temp.next?.next;
		}
		if (auxiliary != null)
		{
			hold = auxiliary;
			auxiliary = auxiliary.next;
			// Separating half nodes
			hold.next = null;
			// Start with first node of next half section
			temp = auxiliary;
			auxiliary = null;
			// Reverse second half nodes 
			while (temp != null)
			{
				hold = temp;
				temp = temp.next;
				hold.next = auxiliary;
				auxiliary = hold;
			}
			// Start with first node of linked list
			temp = this.head;
			// Combine first and second half nodes 
			// in alternate manner
			while (temp != null)
			{
				hold = temp;
				temp = temp.next;
				hold.next = auxiliary;
				if (auxiliary != null)
				{
					hold = auxiliary;
					auxiliary = auxiliary.next;
					hold.next = temp;
				}
			}
		}
	}
}
fun main(args: Array < String > ): Unit
{
	// Create a empty linked lists
	val sll1: SingleLL = SingleLL();
	val 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
	println(" 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
	println(" After folding");
	sll1.display();
	// Test B
	println(" 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
	println(" After folding");
	sll2.display();
}

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