617.合并二叉树

var mergeTrees = function (root1, root2) {
  if (root1 === null && root2 === null) return null;
  if (root1 === null && root2 !== null) return root2;
  if (root1 !== null && root2 === null) return root1;
  let root = new TreeNode(root1.val + root2.val);
  root.left = mergeTrees(root1.left, root2.left);
  root.right = mergeTrees(root1.right, root2.right);
  return root;
};