const inorderTraversal = function(root) {
  const res = []
  pushNode(root, res)
  return res
}

const pushNode = function(node, res) {
  if (!node) {
    return
  }
  node.left && pushNode(node.left, res)
  res.push(node.val)
  node.right && pushNode(node.right, res)
}