Insert node at middle of linked list in kotlin

Suppose we are inserted the following (1, 2, 3, 4, 5, 6, 7) node in a sequence. In this post are implement second approach.

// Kotlin Program for
// Insert linked list element at middle position

// Linked list node
class LinkNode
{
    var data: Int;
    var next: LinkNode ? ;
    constructor(data: Int)
    {
        this.data = data;
        this.next = null;
    }
}
class LinkedList
{
    var head: LinkNode ? ;
    // Class constructors
    constructor()
    {
        this.head = null;
    }
    // Insert node in middle position
    fun insert(value: Int): Unit
    {
        // Create a new node
        val node: LinkNode = LinkNode(value);
        if (this.head == null)
        {
            // First node
            this.head = node;
        }
        else
        {
            var temp: LinkNode ? = this.head;
            var middle: LinkNode ? = this.head;
            // Find the middle node
            while (temp?.next != null && 
              temp.next?.next != null)
            {
                temp = temp.next?.next;
                middle = middle?.next;
            }
            // add node
            node.next = middle?.next;
            middle?.next = 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;
        }
        print("NULL\n");
    }
}
fun main(args: Array < String > ): Unit
{
    val sll: LinkedList = LinkedList();
    // Add node
    sll.insert(1);
    sll.insert(2);
    sll.insert(3);
    sll.insert(4);
    sll.insert(5);
    sll.insert(6);
    sll.insert(7);
    // 1 → 3 → 5 → 7 → 6 → 4 → 2 → NULL
    sll.display();
}

 1 → 3 → 5 → 7 → 6 → 4 → 2 → NULL