比之字形打印二叉树还要少一个动作，那个题是简单，这个是中等？？？ 

        vector<vector<int> > res;
        vector<vector<int> > Print(TreeNode* pRoot) {
            PrintLevel(pRoot, 0);
            return res;
        }
    
        void PrintLevel(TreeNode* pRoot,int level){
            if(pRoot == NULL){
                return;
            }

            if(res.size() < level + 1){
                res.push_back({});
            }
            res[level].push_back(pRoot->val);
            PrintLevel(pRoot->left,level+1);
            PrintLevel(pRoot->right,level+1);
        }