Linked List in kotlin

LinkedList.kt

import java.lang.StringBuilder

fun main() { //testing Linked List
    var linkedList = LinkedList()
    println(linkedList)
    linkedList.preAppend(1).preAppend(2).preAppend(3)
    println(linkedList)
    linkedList.clear()
    linkedList.append(1).append(2).append(3)
    println(linkedList)
    linkedList.deleteWithValue(2)
    println(linkedList)
    linkedList.deleteWithValue(3)
    println(linkedList)
    linkedList.deleteWithValue(2)
    println(linkedList)
    linkedList.deleteWithValue(1)
    println(linkedList)
}


class Node(var data: Int) {
    var next: Node? = null
}

class LinkedList {
    private var head: Node? = null

    fun append(data: Int): LinkedList {
        if (head == null) {
            head = Node(data)
            return this
        }
        var current = head
        while (current?.next != null) {
            current = current?.next
        }
        current?.next = Node(data)
        return this
    }

    fun preAppend(data: Int): LinkedList {
        var newHead = Node(data)
        newHead.next = head
        head = newHead
        return this
    }

    fun clear() {
        head = null
    }

    fun deleteWithValue(data: Int): LinkedList {
        if (head?.data == data) {
            head = head?.next
            return this
        }
        var current = head
        while (current?.next != null) {
            if (current?.next?.data == data) {
                return if (current?.next?.next != null) {
                    current?.next = current?.next?.next
                    this
                } else {
                    current.next = null
                    this
                }
                current = current?.next
            }
        }
        return this
    }


    override fun toString(): String {
        var string: StringBuilder = StringBuilder("{ ")
        if (head == null) {
            string.append("empty Linked List ")
        } else {
            var current = head
            do {
                string.append("${current?.data} ")
                current = current?.next
            } while ((current != null))
        }
        return string.append("}").toString()
    }


}