随机排序

sort(arr) {
  const length = arr.length
  let ret = arr
  for(let i=0; i<length; i++) {
    const x = Math.floor(Math.random() * length)
    const y = Math.floor(Math.random() * length)
    const remp = ret[x]
    ret[x] = ret[y]
    ret[y] = temp
  }
  return ret
}