Sum Root to Leaf Numbers

int sumNumbers(TreeNode *root)
{
return dfs(root, 0);
}
int dfs(TreeNode *root, int sum)
{
if (root == nullptr)return 0;

if (root->left == nullptr && root->right == nullptr)
return sum * 10 + root->val;

return dfs(root->left, sum * 10 + root->val) + dfs(root->right, sum * 10 + root->val);
}