合并二叉树

递归

function mergeTrees(root1: TreeNode | null, root2: TreeNode | null): TreeNode | null {
  if (root1 === null) return root2
  if (root2 === null) return root1

  const merged = new TreeNode(root1.val + root2.val)
  merged.left = mergeTrees(root1.left, root2.left)
  merged.right = mergeTrees(root1.right, root2.right)
  return merged
}