Binary Tree Level Order Traversal [LeetCode]

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

return its level order traversal as:

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

Solution:

 1     vector<vector<int> > levelOrder(TreeNode *root) {
 2         vector<vector<int> > nums;
 3         if(root == NULL)
 4             return nums;
 5         vector<TreeNode *> level;
 6         level.push_back(root);
 7         
 8         while(true){
 9             if(level.size() == 0 )
10                 break;
11             vector<int> level_num;
12             vector<TreeNode *> new_level;
13             for(auto item : level) {
14                 level_num.push_back(item->val);
15                 if(item->left != NULL)
16                     new_level.push_back(item->left);
17                 if(item->right != NULL)
18                     new_level.push_back(item->right);
19             }
20             nums.push_back(level_num);
21             level = new_level;
22         }
23         return nums;
24     }