Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector > levelOrderBottom(TreeNode *root) {
        vector > v;
        int dep=0;
        dfs(root,dep,v);
        reverse(v.begin(),v.end());
        return v;
    }
private:
    void dfs(TreeNode *root,int depth,vector > &v)
    {
        if(root==NULL)
            return ;
        if(depth==v.size())
            v.push_back(vector());
        v[depth].push_back(root->val);
        dfs(root->left,depth+1,v);
        dfs(root->right,depth+1,v);
    }
};