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() == 0 )
                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;
    }