public void dropCourseFromAll(String course) {
		if (root == null) {
			System.out.println("Roster is empty, can't drop course " + course);
			return;
		}

		Queue<StudentNode> queue = new LinkedList<StudentNode>();
		queue.add(root);

		while (queue.size() != 0) {
			StudentNode temp = queue.remove();

			if (temp.courses.contains(course)) {
				temp.courses.remove(course);
			}

			if (temp.left != null) {
				queue.add(temp.left);
			}
			if (temp.right != null) {
				queue.add(temp.right);
			}
		}
	}