Sum of smaller elements of nodes in a linked list in kotlin

Kotlin program for Sum of smaller elements of nodes in a linked list. Here problem description and other solutions.

/*
    Kotlin program for
    Sum of smallest element of linked list
*/
// Linked list node
class LinkNode
{
	var a: Int;
	var b: Int;
	var next: LinkNode ? ;
	constructor(a: Int, b: Int)
	{
		this.a = a;
		this.b = b;
		this.next = null;
	}
}
class SingleLL
{
	var head: LinkNode ? ;
	var tail: LinkNode ? ;
	constructor()
	{
		this.head = null;
		this.tail = null;
	}
	fun insert(a: Int, b: Int): Unit
	{
		val node: LinkNode = LinkNode(a, b);
		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.a + "," + temp.b + ") →");
			// Visit to next node
			temp = temp.next;
		}
		println(" NULL");
	}
	// Find the sum of smaller elements of 
	// every node in a linked list
	fun nodeSum(): Int
	{
		var result: Int = 0;
		var temp: LinkNode ? = this.head;
		if (temp == null)
		{
			println("Empty linked list");
		}
		else
		{
			// Iterate the linked list
			// And 
			// Sum of smallest key in every node of linked list
			while (temp != null)
			{
				if (temp.a > temp.b)
				{
					// When key b is small      
					result += temp.b;
				}
				else
				{
					// When key a is small     
					result += temp.a;
				}
				// Visit to next node
				temp = temp.next;
			}
		}
		return result;
	}
}
fun main(args: Array < String > ): Unit
{
	val sll: SingleLL = SingleLL();
	// Add pair
	// (3,9) → (11,4) → (3,3) → (6,1) → (2,9) → (4,7) → NULL
	sll.insert(3, 9);
	sll.insert(11, 4);
	sll.insert(3, 3);
	sll.insert(6, 1);
	sll.insert(2, 9);
	sll.insert(4, 7);
	println(" Linked List");
	sll.display();
	println(" Result : " + sll.nodeSum());
}

Output

 Linked List
 (3,9) → (11,4) → (3,3) → (6,1) → (2,9) → (4,7) → NULL
 Result : 17