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

         vector<vector<int> > res;

         vector<TreeNode *> layer;

         bool l2r = true;

         layer.push_back(root);

         while (!layer.empty()) {

             vector<TreeNode *> nextLayer;

             vector<int> path;

             for (auto node : layer) {

                 if (!node)

                     continue;

                 path.push_back(node->val);

                 nextLayer.push_back(node->left);

                 nextLayer.push_back(node->right);

             }

             if (!l2r) {

                 for (int i = ; (i << ) < path.size(); i++)

                     swap(path[i], path[path.size() - i - ]);

             }

             if (!path.empty())

                 res.push_back(path);

             layer = nextLayer;

             l2r = !l2r;

         }

         return res;

 }