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},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

Solution:

     vector<vector<int> > levelOrder(TreeNode *root) {
         vector<vector<int> > nums;
         if(root == NULL)
             return nums;
         vector<TreeNode *> level;
         level.push_back(root);

         while(true){
             if(level.size() ==  )
                 break;
             vector<int> level_num;
             vector<TreeNode *> new_level;
             for(auto item : level) {
                 level_num.push_back(item->val);
                 if(item->left != NULL)
                     new_level.push_back(item->left);
                 if(item->right != NULL)
                     new_level.push_back(item->right);
             }
             nums.push_back(level_num);
             level = new_level;
         }
         return nums;
     }