A simple algorithm for finding frequent elements in streams and bags

We present a simple, exact algorithm for identifying in a multiset the items with frequency more than a threshold θ. The algorithm requires two passes, linear time, and space 1/θ. The first pass is an on-line algorithm, generalizing a well-known algorithm for finding a majority element, for identifying a set of at most 1/θ items that includes, possibly among others, all items with frequency greater than θ.