BT_LevelOrder_Rec.cpp

class Solution {
private:
    void dfs(vector<vector<int>>& res, TreeNode* node, int level){
        int len = res.size();
        //How to maintain the current layer list is critical here.
        if(res.empty() || len < level + 1)
            res.push_back(vector<int>());
        res[level].push_back(node->val);
        if(node->left) dfs(res, node->left, level+1);
        if(node->right) dfs(res, node->right, level + 1);
    }
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if(!root) return res;
        dfs(res, root, 0);
        return res;
    }
};