text:  class Solution {
  public:
      vector<int > res;
      void mid_order(TreeNode* root) {
          if (root == NULL) return;
          mid_order(root->left);
          res.push_back(root->val);
          mid_order(root->right);
      }
      vector<int> XXX(TreeNode* root) {
          mid_order(root);
          return res;
      }
  };
  
  