When the officer pursued Sam after retrieving the marijuana, Sam attempted to elude the officer, exited his car and ran into an apartment building. The officer followed Sam into the apartment building, without a warrant. The officer arrested Sam inside the building, also without a warrant. The arrest is:

The arrest is: Invalid

Explanation:

In the American constitution, there is what is called the 4th Amendments which guarantees every American citizens his or her absolute rights always. For Sam to must have ran into a building, it is now assumed that he is within his legal right to not be disturbed.

Any entrance into the building demands a warrant. The police officer going ahead to arrest him without warrant is illegal and invalid.