function levelOrder(root) {
  const queue = [root]
  const list = []
  while (queue.length) {
    const node = queue.shift()
    list.push(node.val)
    node.left && queue.push(node.left)
    node.right && queue.push(node.right)
  }
  return list
}

const list = []

// 前序
function preOrder(root) {
  if (root === null) return;
  list.push(root.val)
  preOrder(root.left)
  preOrder(root.right)
}
// 中序
function preOrder(root) {
  if (root === null) return;
  preOrder(root.left)
  list.push(root.val)
  preOrder(root.right)
}
// 后序
function preOrder(root) {
  if (root === null) return;
  preOrder(root.left)
  preOrder(root.right)
  list.push(root.val)
}