LeetCode problem : Binary Tree Level Order Traversal II

Problem 

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],
]

Code

vector<vector<int> > levelOrderBottom(TreeNode *root) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    map<TreeNode *,int> level;
    deque<TreeNode *> Q;
    int levelNum = 0;
    vector<vector<int> > result;
    if(root){
        Q.push_back(root);
        level[root] = levelNum;
        if(result.size() == levelNum)
        result.resize(2*levelNum + 1);
        result[levelNum].push_back(root->val);
    }
    while(!Q.empty()){
        TreeNode *node = Q.front();
        Q.pop_front();
        if(node->left){
            levelNum = level[node] + 1;
            Q.push_back(node->left);
            level[node->left] = levelNum;
            if(result.size() == levelNum)
                result.resize(2*levelNum + 1);
            result[levelNum].push_back(node->left->val);
        }
        if(node->right){
            levelNum = level[node] + 1;
            Q.push_back(node->right);
            level[node->right] = levelNum;
            if(result.size() == levelNum)
                result.resize(2*levelNum + 1);
            result[levelNum].push_back(node->right->val);
        }
    }
    if(result.empty())
        return result;
    vector<vector<int> > tmp(result.begin(),result.begin() + levelNum +1);
    vector<vector<int> > tmp1(tmp.rbegin(),tmp.rbegin() + levelNum +1);
    return tmp1;
}