set<int> s;

     int findSecondMinimumValue(TreeNode* root) {

         dfs(root);

         vector<int> v;

         for (set<int>::iterator it = s.begin(); it != s.end(); it++) {

             v.push_back(*it);

         }

         sort(v.begin(), v.end());

         return v.size() >  ? v[] : -;

     }

     void dfs(TreeNode *root)

     {

         if (root == NULL)

             return ;

         s.insert(root->val);

         dfs(root->left);

         dfs(root->right);

     }