leetcode #107 in cpp

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,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

Code:

/** * 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) {        deque<vector<int>> res1;        vector<vector<int>> res2;         if(!root) return res2;         deque<TreeNode*> nodes;        nodes.push_front(root);                 while(!nodes.empty()){            vector<int> member;            int cap = nodes.size();            while(cap>0){                                TreeNode*temp = nodes.front();                nodes.pop_front();                                member.push_back(temp->val);                                if(temp->left) nodes.push_back(temp->left);                if(temp->right) nodes.push_back(temp->right);                cap--;            }            res1.push_back(member);        }        while(!res1.empty()){            res2.push_back(res1.back());            res1.pop_back();        }        return res2;    }};