请将下面 C++ 实现的深度优先搜索（DFS）代码补充完整，横线处应填入（）。
struct TreeNode {
int val;
TreeNode* left;
TreeNode* right;
TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};
void dfs(TreeNode* root, vector<int>& result) {
if (root == nullptr) return;
____________
}
- A. ```cpp
result.push_back(root->val);
dfs(root->left);
dfs(root->right);
- **B.** ```cpp
result.push_back(root->left->val);
dfs(root->right);
dfs(root->left);
- C. ```cpp
result.push_back(root->left->val);
dfs(root->left);
dfs(root->right);
- **D.** ```cpp
result.push_back(root->right->val);
dfs(root->right);
dfs(root->left);