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);
      }