code: func RandomString(n int) string {
  	r := make([]byte, n)
  	for i := 0; i < n; i++ {
  		n, err := rand.Int(rand.Reader, big.NewInt(int64(len(letters))))
  		// 没那么惨吧
  		if err != nil {
  			continue
  		}
  		r[i] = letters[n.Int64()]
  	}
  	return string(r)
  }
docs: // RandomString random a string