生成指定长度的随机字符串

extension String{

    static let random_str_characters = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

    static func randomStr(len : Int) -> String{
        var ranStr = ""
        for _ in 0..<len {
            let index = Int(arc4random_uniform(UInt32(random_str_characters.count)))
            ranStr.append(random_str_characters[random_str_characters.index(random_str_characters.startIndex, offsetBy: index)])
        }
        return ranStr
    }

}