Binary Tree Level Order Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> res;
        dfs(root, 0, res);
        reverse(res.begin(), res.end());
        return  res;
    }
private:
    void dfs(TreeNode *r, int level, vector<vector<int>> &ret){
        if(r == NULL)   return;

        if(level == ret.size())
            ret.push_back(vector<int>());

        ret[level].push_back(r->val);

        dfs(r->left, level + 1, ret);
        dfs(r->right, level + 1, ret);
    }
};