Leetcode: Binary Tree Level Order Traversal in C++

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: 
void helper(TreeNode *root, int level, vector<vector<int> >& result)
    {
        if(!root)
            return;
        if(result.size()==level)
        {
            vector<int> tmp;
            tmp.push_back(root->val);
            result.push_back(tmp);
        }
        else
           result[level].push_back(root->val);
        if(root->left)
           helper(root->left, level+1, result);
        if(root->right)
           helper(root->right, level+1, result);
    }
    vector<vector<int> > levelOrder(TreeNode *root) {
        vector<vector<int> > result;
        if(!root)
        return result;
        helper(root, 0, result);
        return result;
    }