以下代码可以正确地按层换行输出二叉树的节点值。
void printByLevel(TreeNode* root) {
if (!root) return;
queue<TreeNode*> q;
q.push(root);
while (!q.empty()) {
for (int i = 0; i < q.size(); ++i) {
TreeNode* cur = q.front();
q.pop();
cout << cur->val << " ";
if (cur->left) q.push(cur->left);
if (cur->right) q.push(cur->right);
}
cout << endl;
}
}