@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();
}
