What would happen if US pulls out of SK and Japan?

It is highly unlikely that the US would pull out of South Korea and Japan at this time. That being said, they have gradually decreased the number of standing military personnell in these countries since the end of the Cold War, so their military commitment to the region (being East Asia) has decreased somewhat.

However, they still maintain a HUGE influence on this region, centered around the Korean Peninsula, with the major contenders for influence being China, Russia, and of course the US of A. This is evident in the "Six Party Talks", a multilateral security forum aiming to lower tensions on the peninsula specifically, and the region as a whole. Basically, a complete military retreat from the region would no longer legitimise American claims for a continued influence in this political sphere, and both Russia and China would jump to fill the power gap (that does NOT necessairly mean war/military action).

Now, let's look at this from an economical perspective. Japan was JUST recently overtaken by China as the second largest economy in the world, and is still FAR behind the US in this regard. Japan and South Korea's economies are larger than the Chinese economy when put together by a fair ammount. As such, China cannot dictate regional policy purely based on economic pressure/extortion, nor would they want to as they are interested in continued trade and economic growth, especially with the US.

I also see a lot of people saying stuff like "North Korea could crush South Korea with artillery and nuke Japan, and then kill everyone who invaded because they have such a large army, etc". Ok, so, what is the overall goal of the North Korean regime? To survive. This is their ONLY goal; not the spread of communism, not the fight against capitalism, nor any other ideological struggle. They simply want to carry on and be left alone (except for the continued shipments of food/oil/medicine.) North Korea is fucked if they invade South Korea, the regime would NOT survive such a war, and the region would suffer devastating consequences. Contrary to popular belief, North Korea DOES NOT possess any dependable means of delivery of a nuclear weapon, nor is there any empirical evidence of them actually managing to produce a nuclear weapon. Yes, I am aware a nuclear explosion was recorded a little while back by American Seismological instruments, but this was a relatively small explosion, much smaller than the Nagasaki bomb, and indeed it was downclassed to that of a nuclear device, NOT a bomb. As for means of delivery, North Korea has made several attempts to launch a satellite into orbit, failing miserably every time. Yes, they do possess some missile systems, but no, these are not accurate enough, nor do they have the range to really harm Japan, unless they get lucky (kinda like V1 rockets ruing WW2). In a conventional war, North Korea has a superior lead in both standing army, tanks, guns, ships, etc ON PAPER. Did you know most North Korean vessels are actually mini-submarines or gunboats/patrol boats? Did you know the standing army is largely due to the millions of reservists they have conscripted? And have you even considered the technoligical deficit they would find themselves in when going against the most advanced war machines on the planet?

Finally, China would NEVER engange in a war that North Korea had started. They didn't do so in 1950, they wouldn't do so now. They were even instrumental in the division of the country along the 38th parallel