const levelOrder = function(root) {
  if(!root) return []
  const res = []
  const stack = []
  stack.push(root)
  while(stack.length > 0){
      const cur = stack.shift()
      res.push(cur.val)
      cur.left && stack.push(cur.left)
      cur.right && stack.push(cur.right)
  }

  return res
};