节点最长距离

function findLongest(root) {
  if (!root) {
    return 0
  }

  const leftDepth = getDepth(root.left)
  const rightDepth = getDepth(root.right)

  const leftLongest = findLongest(root.left)
  const rightLongest = findLongest(root.right)

  return Math.max(leftDepth + rightDepth + 1, leftLongest, rightLongest)
}

function getDepth(node) {
  if (!node) {
    return 0
  }
  const left = getDepth(node.left)
  const right = getDepth(node.right)

  return Math.max(left, right) + 1
}