fun reverseList(head: ListNode?): ListNode? {

    if (head == null) return null

​

    var currentNode = head

    var previousNode: ListNode? = null

    var reversedNode: ListNode? = null

​

    while (currentNode != null) {

        val next = currentNode.next

        if (next == null) {

            reversedNode = currentNode

        }

​

        currentNode.next = previousNode

        previousNode = currentNode

        currentNode = next

    }

    return reversedNode

}