For DC, as correctly stated, the solutions are either a DPDT switch or the contacts built in to some commercially available turntables which basically have two semicircular static tracks onto which spring loaded contacts attached to the turntable bridge make contact. When the turntable gets halfway round, contact is momentarily cut and then reversed as the spring loaded contacts connect with the opposite semicircular track.

For DCC, the polarity still needs to be switched to avoid a short circuit when part of the locomotive is on the turntable bridge and part on the surrounding track. Again, a DPDT switch can be used for this but it will affect sound if fitted. A better solution is to use a reverse loop module or two frog juicers between the main power bus and the turntable bridge which will deal with it automatically without interrupting sound.

If the system is DCC, polarity is irrelevant, as the track voltage is AC and the loco goes in the direction is is told by the commands sent to it.

Electrical polarity (DC or DCC) switching issues only occur when you have triangles or balloon loops in your track.

I’m guessing you have scratch built the turntable? (in which case, well done!) All commercially available turntables and kits that I have seen have a built in way to reverse the polarity as it turns, usually by having the power supplied through a set of contacts that run over two fixed strips, each strip going only halfway around, so that when the table is turned the contacts “change strips” and thus change polarity.