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<vector<int> > levelOrderBottom(TreeNode *root) {
        vector<vector<TreeNode*> > list;
        vector<vector<int> > result;
        if(root==NULL)
            return result;
        vector<TreeNode*> level;
        level.push_back(root);
        list.push_back(level);
        while(level.size()>0)
        {
            vector<TreeNode*> newlevel;
            for(int i=0;i<level.size();i++)
            {
                if(level[i]->left!=NULL) newlevel.push_back(level[i]->left);
                if(level[i]->right!=NULL) newlevel.push_back(level[i]->right);
            }
            list.push_back(newlevel);
            level=newlevel;
        }
        
        for(int i=0;i<list.size()-1;i++)
        {
            vector<int> v;
            for(int j=0;j<list[list.size()-i-2].size();j++)
                v.push_back(list[list.size()-i-2][j]->val);
            result.push_back(v);
        }
        return result;
    }
};