请将下列 DFS 代码补充完整，横线处应填入：
void dfs(TreeNode* root, vector<int>& result) {
if (root == nullptr) return;
________ // 在此处填入
}
- A. result.push_back(root->val); dfs(root->left); dfs(root->right);
- B. result.push_back(root->left->val); dfs(root->right); dfs(root->left);
- C. result.push_back(root->left->val); dfs(root->left); dfs(root->right);
- D. result.push_back(root->right->val); dfs(root->right); dfs(root->left);
正确答案：A