ios 跳转到app store

 NSString *str = [NSString stringWithFormat:@"https://itunes.apple.com/cn/app/zhang-xiao-tong-lao-shi-ban/id562675943?mt=8"];
        //https://itunes.apple.com/cn/app/zhang-xiao-tong-jia-zhang-ban/id562206389?mt=8  家长版
        //https://itunes.apple.com/cn/app/zhang-xiao-tong-lao-shi-ban/id562675943?mt=8  教师版
        [[UIApplication sharedApplication] openURL:[NSURL URLWithString:str]];