The high priesthood had been for some time in a certain sense divided between Zadok and Abiathar. (See the 1 Kings 1:8 note). Henceforth, Zadok became sole high priest.
Related Commentaries of 1 Kings 2:35
1 Kings 2:35
35 And the king put Benaiah the son of Jehoiada in his room over the host: and Zadok the priest did the king put in the room of Abiathar.