void levelOrder(TreeNode* root) {
    if (!root) return;
    queue<TreeNode*> que;
    que.push(root);
    while(!que.empty()) {
        int size = que.size();
        for (int i = 0; i < size; i++) {
            TreeNode* cur = que.front();
            que.pop();
            //对弹出节点作相应操作
            if (cur->left) {
                que.push(cur->left);
            }
            
            if (cur->right) {
                que.push(cur->right);
            }
        }
        //这里可以计算深度
    }
}