下列代码实现树的深度优先遍历，横线处应填入：
while (!st.empty()) {
TreeNode* node = st.top(); st.pop();
cout << node->val << " ";
if (node->right) st.push(node->right);
________
}
- A. if (node->left) st.push(node->left);
- B. if (node->left) st.pop(node->left);
- C. if (node->left) st.front(node->left);
- D. if (node->left) st.push(node->right);
正确答案：A