are electrolytic capacitors bilateral or unilateral element?
The current through an electrolytic capacitor can flow in both directions, in that sense it is no different from a non-polarized capacitor.

The main difference is that the voltage across an electrolytic capacitor must not be reversed. That is, the voltage between the + terminal and the - has to be larger than or equal to zero volts at all times. Though, a small reverse voltage can sometimes be tolerated.