题目：Binary Tree Inorder Traversal

二叉树的中序遍历，和前序、中序一样的处理方式，代码见下：

1 struct TreeNode { 2     int            val; 3     TreeNode*    left; 4     TreeNode*    right; 5     TreeNode(int x): val(x), left(NULL),right(NULL) {} 6 }; 7  8 vector
     
       preorderTraversal(TreeNode *root) //非递归的中序遍历(用栈实现) 9 {10     if (NULL == root) {11         return vector
      
       ();12     }13 14     stack
       
         tree_stack;15     vector
        
          tree_vector;16 17     TreeNode *pTemp = root;18     while (pTemp || !tree_stack.empty()) {19         while (pTemp) {20             tree_stack.push(pTemp);21             pTemp = pTemp->left;22         }23         if (!tree_stack.empty()) {24             pTemp = tree_stack.top();25             tree_vector.push_back(pTemp->val);26             tree_stack.pop();27             pTemp = pTemp->right;28         }29     }30     return tree_vector;31 }