vector<vector<int>> levelOrder(TreeNode* root) {
            vector<vector<int>>res;
            vector<int>level;
            if(root == NULL)
              return res ;
         
            TreeNode* cur ;
            queue<TreeNode*>q;
            q.push(root);
            q.push(NULL);  
            while(!q.empty()){
                cur = q.front();
                q.pop();
                if(cur!=NULL){
                    level.push_back(cur->val);
                    if(cur->left!=NULL) q.push(cur->left);
                    if(cur->right!=NULL)q.push(cur->right);  
                 }
                else {
                    res.push_back(level);
                    level.clear();
                    if( !q.empty()&&q.front()!=NULL)
                      q.push(NULL);  
                }
            }
            return res;
    }