public static void preOrder(Node root) {
    if (root == null) return;

    System.out.print(root.data + " ");
    preOrder(root.left);
    preOrder(root.right);
}

public static void preOrder(Node root) {
    if (root == null) return;

    Stack<Node> stack = new Stack<>();
    stack.push(root);

    while (!stack.isEmpty()) {
        Node current = stack.pop();
        System.out.print(current.data + " ");
				
        if (current.right != null) {
            stack.push(current.right);
        }

        if (current.left != null) {
            stack.push(current.left);
        }
    }
}