二叉树的直径

function diameterOfBinaryTree(root: TreeNode | null): number {
  let max = 0

  const maxDepth = (root: TreeNode | null): number => {
    if (root === null) return 0

    const left = maxDepth(root.left)
    const right = maxDepth(root.right)
    max = Math.max((left + right), max)
    return Math.max(left, right) + 1
  }

  maxDepth(root)

  return max
}