poll

Removes and returns the highest-priority (smallest) element. Returns null if the queue is empty.

Example:

    var pq = new PriorityQueue<string> ((a, b) => {
        return strcmp (a, b);
    });
    pq.add ("b");
    pq.add ("a");
    assert (pq.poll () == "a");

Returns:

the smallest element, or null if empty.