text:  class Solution {
  public:
      vector<int>temp;
      void dfs(TreeNode* root){
           if(!root)return;
          dfs(root->left);
          temp.push_back(root->val);
          dfs(root->right);
      }
      vector<int> XXX(TreeNode* root) {
          dfs(root);
          return temp;
      }
  };
  
  