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;
_________________
}result.push back(root->val);
dfs(root->left);
dfs(root->right);
result.push back(root->left->val);
dfs(root->right);
dfs(root->left);
result.push back(root->left->val);
dfs(root->left);
dfs(root->right);
result.push back(root->right->val);
dfs(root->right);
dfs(root->left);