Efficiently storing and retrieving likes

In my Firebase database I have posts and then authenticated users can "like" posts. How can I efficiently get the number of likes a post has received. I know using MongoDB I can add/remove the user's id to a list and then use a MongoDB function to get the length of it very quickly and set that equal to the likes amount, but I'm not suer how to do that using Firebase. I could also add/remove it to the list and increment a likeCount variable, but that seems like it would cause concurrency issues unless Firebase has a function for that. What functions can I call to best handle this and scale well? Thanks in advance!