@Override public String toString() { if (root == null) return "NULL"; StringBuilder sb = new StringBuilder(); LinkedList<BTNode> queue = new LinkedList<BTNode>(); queue.push(root); BTNode tem = null; while ((tem = queue.poll()) != null) { for (BTNode node : tem.children) { if (!node.isNull()) queue.offer(node); } sb.append(tem.toString() + "\n"); } return sb.toString(); }