laravel引用Validator报重定向错误？

Question

新建了LoginRequest

class LoginRequest extends Request
{
    public function authorize()
    {
        return false;
    }

    public function rules()
    {
        return [
            "username" => "required",
            "password" => "required",
            "captcha" => "required|captcha"
        ];
    }

    public function messages()
    {
        return [
            "username.required" => "请输入用户名!",
            "password.required" => "请输入密码!",
            "captcha.required" => "请输入验证码!",
            "captcha.captcha" => "验证码错误，请重试!"
        ];
    }
}

怎样在LoginController控制器中应用？

class LoginController extends CommonController
{
    public function login(LoginRequest $request)
    {
        if ($request->isMethod('post')){
            $input = $request->all();
            $user = User::first();
            if ($user->username == $input['username'] && Crypt::decrypt($user->password) == $input['password']){
                $request->session()->put('username', $input['username']);
                return \redirect()->action('Admin\IndexController@index');
            }else{
//                $validator->errors()->add('errors', '用户名或密码错误!');
                return Redirect::back()->withErrors($validator);
            }
        }else{
            return view('Admin/login');
        }
    }
}

这样报重定向错误