pre_order_traversal

private void iterativePreOrder(Node node) {
        if (node == null) return;
        // create an empty stack and push root to it
        Stack<Node> stack = new Stack<>();

        while (node != null || !stack.isEmpty()){
            if (node != null){
                System.out.print(node.data + " ");
                stack.push(node);
                node = node.left;
            }else{
                node = stack.pop();
                node = node.right;
            }
        }
    }