class Node(val data: Int, var next: Node? = null) {
    
    override fun toString(): String {   
        
        // displays stack in reverse order
        // where top of stack is the right-most element
        var output = if (next != null) "$next, " else ""   
        output += "$data"  
        
        return output       
    }
}
class Stack {       
    private var top: Node? = null
    
    fun push(data: Int) {
        val node = Node(data)
        node.next = top
        top = node
    }
    fun pop(): Int? {
        val data = top?.data
        top = top?.next
        return data
    }
    override fun toString(): String { 
        var output = "Stack: "
        output += if (top != null) "[$top]" else "[]"
        return output + " <-top"
    }
}
fun main() {
    val stack = Stack()
    println(stack)  
    println("${stack.pop()} popped")
    stack.push(3)
    stack.push(2)
    stack.push(1)
    println(stack)  
    println("${stack.pop()} popped")
    println(stack) 
    println("${stack.pop()} popped")
    println(stack)
    println("${stack.pop()} popped")
    println(stack)
    println("${stack.pop()} popped")
    println(stack)
}