到底是 prep 定 adv

Adverb!!!!

An adverb modifies a verb, an adjective or another adverb. In your sentence, it modifies the verb 'come'.

A preposition always goes in front of a noun or pronoun. It descripts the relationship between the noun and other words in the sentence.
e. g. The book is in the drawer.

The 'in' in 'please come in' is an adverb of place. It tells us 'come' to where

“In” can be used in the following way:~

(1) as an adverb (without a following noun)
Please come in.

(2) as a preposition (followed by a noun)
Please come in the morning.

Come 是 intransitive verb (no object)
Please 只不過是 interjection. 禮貌些

Preposition
Expressing the situation of someone or something that is or appears to be enclosed or surrounded by something else(表達某人或某物被另物包或圍着的情況時是prep)例: "living in Hong Kong".

2013-07-29 20:03:12 補充：
Adverb
Expressing movement with the result that someone or something becomes enclosed or surrounded by something else(表達令某人或某物變作被另物包或圍着的動作時是adv)例: "come in"; "bring it in".
Therefore, Please come in 中的“in" 是屬於 adverb.