Add two numbers represented by linked lists in kotlin

Kotlin program for Add two numbers represented by linked lists. Here problem description and explanation.

/*
    Kotlin program for
    Add two numbers represented by 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 ? ;
	var tail: LinkNode ? ;
	constructor()
	{
		this.head = null;
		this.tail = null;
	}
	fun insert(data: Int): Unit
	{
		val node: LinkNode = LinkNode(data);
		if (this.head == null)
		{
			// Add first node
			this.head = node;
		}
		else
		{
			// Add node at the end position
			this.tail?.next = node;
		}
		// New last node
		this.tail = 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");
	}
	// Reverse a single linked list
	fun reverse(): Unit
	{
		var prev: LinkNode ? = null;
		var temp: LinkNode ? = this.head;
		while (temp != null)
		{
			// New head node
			this.head = temp;
			// Visit to next node
			temp = temp.next;
			// Change node link to pervious node
			this.head?.next = prev;
			// Get the newly head node
			prev = this.head;
		}
	}
	fun addition(b: SingleLL): SingleLL
	{
		// Reverse both linked lists
		this.reverse();
		b.reverse();
		// Define some variables
		val result: SingleLL = SingleLL();
		var carry: Int = 0;
		var x: LinkNode ? = this.head;
		var y: LinkNode ? = b.head;
		// Iterate the both linked lists
		while (x != null || y != null)
		{
			if (x != null)
			{
				// Add node data of x
				carry += x.data;
				// Visit to next node
				x = x.next;
			}
			if (y != null)
			{
				// Add node data of y
				carry += y.data;
				// Visit to next node
				y = y.next;
			}
			// Add resultant node
			result.insert(carry % 10);
			//  Remove last digit of carry
			carry = carry / 10;
		}
		// Add remaining carry digits
		while (carry != 0)
		{
			// Add resultant node
			result.insert(carry % 10);
			// Remove last digit of carry
			carry = carry / 10;
		}
		// Reverse linked list
		b.reverse();
		this.reverse();
		result.reverse();
		// Return result
		return result;
	}
}
fun main(args: Array < String > ): Unit
{
	val a: SingleLL = SingleLL();
	val b: SingleLL = SingleLL();
	// 5112
	a.insert(5);
	a.insert(1);
	a.insert(1);
	a.insert(2);
	// 99999
	b.insert(9);
	b.insert(9);
	b.insert(9);
	b.insert(9);
	b.insert(9);
	// 5112 + 99999
	// ------------
	// 105111
	val c: SingleLL = a.addition(b);
	c.display();
}

Output

 1 → 0 → 5 → 1 → 1 → 1 → NULL