[leetcode] Binary Tree Level Order Traversal II

From : https://leetcode.com/problems/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]
]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

/**
 * 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) {
            queue<TreeNode> box;
            box.push(*root);
            int num=1, newNum=0;
            while(num) {
                vector<int> cur;
                while(num) {
                    TreeNode node = box.front();
                    box.pop();
                    num--;
                    cur.push_back(node.val);
                    if(node.left)  {box.push(*node.left); newNum++;}
                    if(node.right) {box.push(*node.right);newNum++;}
                    
                }
                res.insert(res.begin(), cur);
                num = newNum;
                newNum = 0;
            }
        }
        return res;
    }
};