left_tree = inorderTraversal(root->left); right_tree = inorderTraversal(root->right); res_tree.insert(res_tree.end(), left_tree.begin(), left_tree.end());

这段代码是二叉树中序遍历的代码，将左子树和右子树的遍历结果分别存储在`left_tree`和`right_tree`中，然后将`left_tree`中的元素插入到结果数组`res_tree`的末尾。具体来说，`res_tree.insert(res_tree.end(), left_tree.begin(), left_tree.end());`这一行代码的作用是将`left_tree`中的所有元素插入到`res_tree`的末尾。

这段代码的完整作用是遍历一个二叉树，将其中序遍历的结果存储在`res_tree`中。

示例：

#include <iostream>
#include <vector>

struct TreeNode {
  int val;
  TreeNode *left;
  TreeNode *right;
  TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
public:
    std::vector<int> inorderTraversal(TreeNode* root) {
        std::vector<int> res_tree;
        if (root == nullptr) {
            return res_tree;
        }
        std::vector<int> left_tree, right_tree;
        left_tree = inorderTraversal(root->left);
        right_tree = inorderTraversal(root->right);
        res_tree.insert(res_tree.end(), left_tree.begin(), left_tree.end());
        res_tree.push_back(root->val);
        res_tree.insert(res_tree.end(), right_tree.begin(), right_tree.end());
        return res_tree;
    }
};

int main() {
  TreeNode *root = new TreeNode(1);
  root->right = new TreeNode(2);
  root->right->left = new TreeNode(3);

  Solution s;
  std::vector<int> res = s.inorderTraversal(root);

  for (auto i : res) {
    std::cout << i << " ";
  }

  return 0;
}

输出结果为：

1 3 2