You Do Need power plug adapter while traveling from Swaziland to Yemen.

Here is Why You Need An Adapter

The outlet and plug type used in Yemen are A, D and G and in Swaziland is M. This is how they look like.

Swaziland

In Swaziland, outlets of type M are in use.

Type M

Yemen

In Yemen, outlets of type A, D and G are in use.

Type A

Type D

Type G