have a good/hard/difficult time (in )doing sth这里的in可以省略的,因为in是介词,介词后面通常情况下可加名词或动名词（doing）
类似的还有have an interest (in) doingsth have fun (in) doing sth have trouble (in) doing sth 等
简单来说就是have+（形容词）名词+(in)doing sth,当名词可数时前面有a/an
可以当成短语来记忆