vector<int> inorderTraversal(TreeNode* root) {

         // 非递归实现，借助栈

         vector<int> ans;

         if( root == NULL)

             return ans;

         stack< const TreeNode *> s;

         const TreeNode* p = root;

         while( !s.empty()  || p != NULL )

         {

             if( p != NULL )

             {

                 s.push(p);

                 p = p->left;

             }

             else

             {

                 p = s.top();

                 s.pop();

                 ans.push_back( p->val );

                 p = p->right;

             }

         }

         return ans;