Devise a small memory streaming algorithm to determine if |A| = 1.

For a stream of insertions and deletions, recall that x[j] = #insertions - #deletions of element j.
Given such a stream satisfying x[j] >= 0 for all elements j, let

A = { j : x[j] > 0 }

Determining whether A is empty is easy: just check if the sum of all x[j]'s equals 0 (which is easily doable in a stream).

Problem: devise a small memory streaming algorithm to determine if |A| = 1.

Extensions: What about higher sizes of A? What if the promise is not satisfied and we define A to be the set of j's with x[j] not equal to 0.