fun reverseNonEmptyListRecursive(node: ListNode): ListNode {
    val nextNode = node.next
    return if (nextNode == null) node
    else {
        // The recursion stack frame will be pushed until it reached the tail node
        val reversed = reverseNonEmptyListRecursive(nextNode)
        // then it will start popping

        // reversing process
        nextNode.next = node

        // this line is to set the reversed list's tail(originally head)'s `next` to `null`
        // for all nodes other than original head,
        // `node.next = null` would be overwritten by the statement
        // `nextNode.next = node` in the one-level shallower recursion stack frame
        node.next = null

        reversed
    }
}
return reverseNonEmptyListRecursive(head)