以下代码可以正确地按层换行输出二叉树的节点值。

1void printByLevel(TreeNode* root) {2   if (!root) return;3 4   queue<TreeNode*> q;5   q.push(root);6 7   while (!q.empty()) {8      for (int i = 0; i < q.size(); ++i) {9        TreeNode* cur = q.front();10        q.pop();11        cout << cur->val << " ";12 13        if (cur->left) q.push(cur->left);14        if (cur->right) q.push(cur->right);15      }16      cout << endl;17   }18}