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

         vector<vector<int>> ans;

         if (root == NULL)

             return ans;

         queue<TreeNode*> q;

         q.push(root);

         vector<int> v;

         while (!q.empty()) {

             int size = q.size();

             for (int i = ; i < size; i++) {

                 TreeNode *node = q.front();

                 q.pop();

                 v.push_back(node->val);

                 if (node->left != NULL)

                     q.push(node->left);

                 if (node->right != NULL)

                     q.push(node->right);

             }

             ans.push_back(v);

             v.clear();

         }

         reverse(ans.begin(), ans.end());

         return ans;

     }