most of them have 100-240v auto select charger. no need to look for a selector to select 100v or 240v, only thing you need is a compatible convertor head, since every country uses a different head. like sg is the 3 pin plug rectangle plug, den some uses 3 pin circle plug & some only 2 pin but the ground is in the middle.
there is a list on the net on different head, can search around.
alternatively, you can get a universal convertor at any electronics/electrical store. or at worst, go to changi airport Boon superstore, sure to have.