Stack on Kotlin

class Stack<T>(private val value: T){
    class Node<T>(val value: T, val prev: Node<T>? = null)
    private var head: Node<T>?
    init {
        head = Node(value)
    }
    fun push(value: T) {
        head = Node(value, head)
    }
    fun pop():T? {
        val tmp = head
        head = head?.prev
        return tmp?.value
    }
}