private String nodeString(IntervalNode node, int level) {
		if (node == null) return "";

		StringBuffer sb = new StringBuffer();
		for (int i = 0; i < level; i++)
			sb.append("\t");
		sb.append(node + "\n");
		sb.append(nodeString(node.getLeft(), level + 1));
		sb.append(nodeString(node.getRight(), level + 1));
		return sb.toString();
	}