class Solution {
private:
    void make_order(TreeNode *cur, int level, vector<vector<int>> &ans)
    {
        if (ans.size() == level)
            ans.resize(level + 1);
        ans[level].push_back(cur->val);
        if (cur->left != NULL)
            make_order(cur->left, level + 1, ans);
        if (cur->right != NULL)
            make_order(cur->right, level + 1, ans);
        return;
    }
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if (root == NULL)
            return ans;
        make_order(root, 0, ans);
        return ans;
    }
};