下面将结点插入二叉搜索树，横线处应填入：
tree_node* insert(tree_node* root, int val) {
if (root == nullptr) return new tree_node(val);
________ // 在此处填入代码
return root;
}
- A. if (val < root->val) root->left = insert(root->left, val); else root->right = insert(root->right, val);
- B. if (val > root->val) root->left = insert(root->left, val); else root->right = insert(root->right, val);
- C. if (val < root->val) root->left = insert(root, val); else root->right = insert(root, val);
- D. if (val > root->val) root->left = insert(root, val); else root->right = insert(root, val);
正确答案：A