请将下列 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.

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);

{{ select(1) }}

• A
• B
• C
• D