code: func GetRandomString(n int, alphabets ...byte) string {
  	const alphanum = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
  	var bytes = make([]byte, n)
  	rand.Read(bytes)
  	for i, b := range bytes {
  		if len(alphabets) == 0 {
  			bytes[i] = alphanum[b%byte(len(alphanum))]
  		} else {
  			bytes[i] = alphabets[b%byte(len(alphabets))]
  		}
  	}
  	return string(bytes)
  }
docs: // GetRandomString generate random string by specify chars.
  // source: https://github.com/gogits/gogs/blob/9ee80e3e5426821f03a4e99fad34418f5c736413/modules/base/tool.go#L58