/** 遍历一棵树，层次遍历。 */
public void traverse() {
if (id == null) {
return;
}
// 执行遍历后动作。
if (childList == null || childList.isEmpty()) return;
int childNumber = childList.size();
for (int i = 0; i < childNumber; i++) {
TreeNode child = childList.get(i);
child.traverse();
}
}
