You could have a balanced data structure and keep inserting into it.

by using an N element max/min heap and evicting the max/min when a new element comes in that's less than the max/greater than the min (note this is also a leetcode problem https://leetcode.com/problems/k-closest-points-to-origin/)

Since no one else got it, I'd start by allocating an infinite array to ingest the stream into.

O(∞)

the post was getting too long to put in all the details, but basically, what I was getting at, imagine the data comes in batches and you have to retain the N largest seen so far