struct TreeNode {

     int            val;

     TreeNode*    left;

     TreeNode*    right;

     TreeNode(int x): val(x), left(NULL),right(NULL) {}

 };

 void Swap(vector<int> &ivec)

 {

     int temp = ;

     int len = ivec.size();

     for (int i = ; i < len/; i ++) {

         temp = ivec.at(i);

         ivec.at(i) = ivec.at(len--i);

         ivec.at(len--i) = temp;

     }

 }

 vector<vector <int> > zigzagLevelOrder(TreeNode *root)

 {

     vector<vector<int> > tree_vector;

     vector<int> ivec;

     queue<TreeNode *> tree_queue;

     if (NULL == root)

         return tree_vector;

     tree_queue.push(root);

     tree_queue.push(NULL);

     int nLevelCount = ;

     while (true) {

         TreeNode *pTemp = tree_queue.front();

         tree_queue.pop();

         if (pTemp == NULL) {

             if (nLevelCount% == ) { //if the num of level is odd, swap the ivec;

                 Swap(ivec);

             }

             tree_vector.push_back(ivec);

             ivec.clear();

             if (tree_queue.empty())

                 break;

             tree_queue.push(NULL);

             nLevelCount ++;

         }

         else {

             ivec.push_back(pTemp->val);

             if (pTemp->left)

                 tree_queue.push(pTemp->left);

             if (pTemp->right)

                 tree_queue.push(pTemp->right);

         }

     }

     return tree_vector;

 }