def enqueue item, priority=0
      raise RangeError.new("priority #{priority} is greater than queue size") if priority > self.size
      @array.push PQNode.new(item, priority)
      @array.sort! { |a, b| a.priority <=> b.priority }
    end
def enqueue item, priority=0
      raise RangeError.new("priority #{priority} is greater than queue size") if priority > self.size
      @array.push PQNode.new(item, priority)
      @array.sort! { |a, b| a.priority <=> b.priority }
    end
Add +:item+ to the queue with +:priority+.
 +:priority+ must be an integer between 0 and queue.size