code: public void push(int element) {
      if (element < 0)
        throw new IllegalArgumentException("Cannot add negative integers to the priority queue");
      assert !this.contains(element);
      this.doImport(element);
      this.pos.set(element, this.heap.size());
      this.heap.push(element);
      assert this.heap.get(this.pos.get(element)) == element;
      this.up(element);
    }
docs: Pushes a new positive element to the queue.
  @param element the element
  @throws IllegalArgumentException if the element to add is negative