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,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

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/** * 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;        if(root == NULL) return res;        queue<TreeNode*>q;        q.push(root);        while(!q.empty()){            vector<int>p;            int _size = q.size();            for(int i = 0; i < _size; i++){            TreeNode *temp = q.front();            q.pop();            p.push_back(temp->val);            if(temp->left)                q.push(temp->left);            if(temp->right)                q.push(temp->right);            }            res.push_back(p);        }       reverse(res.begin(), res.end());        return res;    }};