TreeConstant

public static class Node {
    public Node left, right;
    public Node parent;
    public String data;

    public Node(String data, Node parent) {
        this.data = data;
        this.parent = parent;
    }

    public Node addLeft(String data) {
        left = new Node(data, this);
        return left;
    }

    public Node addRight(String data) {
        right = new Node(data, this);
        return right;
    }
}

public static void main(String[] args) {
/*  0   1   2   3   */
    Node root = new Node("root", null);
    {
        Node n10 = root.addLeft("1,0");
        {
            Node n20 = n10.addLeft("2,0");
            {
                Node n30 = n20.addLeft("3,0");
                Node n31 = n20.addRight("3,1");
            }
            Node n21 = n10.addRight("2,1");
            {
                Node n32 = n21.addLeft("3,2");
                Node n33 = n21.addRight("3,3");
            }
        }
        Node n11 = root.addRight("1,1");
        {
            Node n22 = n11.addLeft("2,2");
            Node n23 = n11.addRight("2,3");
        }
    }
    /* 11 nodes
     *             *
     *           /   \
     *          *     *
     *         / \   / \
     *        /\ /\
     */

    int deep = 0;
    int lastDeep = 0;
    boolean leftRight = false; // false = left, true = right
    boolean upDown = true; // false = up, true = down

    Node current = root;
    do {
        if (upDown == true) {
            System.out.println(current.data);
        } else if (lastDeep == deep) {
            if (leftRight == false) {
                current = current.right;
                System.out.println(current.data);
                deep++;
                upDown = true;
                leftRight = true;
            } else {
                leftRight = (current.parent.right == current);
                current = current.parent;
                lastDeep--;
                deep--;
                upDown = false;
            }
        }

        if (current.left != null && current.right != null)  {
            lastDeep = deep;
            if (upDown == true) {
                current = current.left;
                deep++;
                upDown = true;
                leftRight = false;
            }
        } else {
            leftRight = (current.parent.right == current);
            current = current.parent;
            deep--;
            upDown = false;
        }
    } while (leftRight != true || deep != 0);
}