Is it correct to say "Can I come in if don't have a ticket"?
or,
Which is more common in ordinary life:
A. Can I enter if don't have a ticket?
B. Can I enter if I don't have a ticket?
Is it correct to say "Can I come in if don't have a ticket"?
or,
Which is more common in ordinary life:
A. Can I enter if don't have a ticket?
B. Can I enter if I don't have a ticket?
No, your sentence isn't quite correct...
Can I come in if don't have a ticket?
You're missing the subject of the clause. You need to add another "I" in there, like so:
Can I come in if I don't have a ticket?
Of the two sentences you presented, A is problematic but B is fine, and B is therefore more common in ordinary life.
You can sometimes omit redundant subjects (see here) but not in this case.