Kim Jong-Un did not have surgery during his absence. In fact, there was nothing out of the ordinary about his short hiatus at all. South Korean intelligence officials wholeheartedly dismissed rumors that anything odd happened with Kim Jong-Un, stating that there was "nothing unusual" about the North Korean leader and his health. Since South Korea has many sources with which to monitor its northern neighbor, we can trust that its information is valid.
One crucial piece of information that is missing from South Korea's statements is any explanation about why Kim Jong-Un was absent in the first place. If there was truly nothing unusual about the situation, then South Korean intelligence should have been able to explain in more detail. Another complicating factor is that it is very uncommon for South Korea to issue any statements about the goings-on in North Korea, making their already scant report seem potentially suspicious.