function preOrder(tree1, tree2) {

  if (tree1 == null) {

    return tree2;

  }

  if (tree2 == null) {

    return tree1;

  }

  return tree1.val + tree2.val;

​

  // Do the above on both of their left children

  preOrder(tree1.left, tree2.left)

​

  // Do the above on both of their right children

  preOrder(tree1.right, tree2.right)

}