Arduino Question regarding the resistor
While driving a load with a microcontroller, you may add resistors on either side. This is because the current entering the LED and leaving out of the LED is the same. So if a resistor is used on the high side or low side, it would limit the current. They can be used on either side and either way because they are not polarized.

In such configurations, resistors limit current flowing through an active component (LED in your case). Considering Kirchoff's Law, it doesn't matter if the resistor is on the positive or the negative side of the LED. It would limit the same amount of current irrespective of the positive side or negative side of LED.

More reading: Kirchhoff's Laws