우선순위 큐

//Queue<Integer> queue = new PriorityQueue<>();  //min heap
Queue<Integer> queue = new PriorityQueue<>(Collections.reverseOrder());  //max heap

queue.add(2);
queue.add(3);
queue.add(7);
queue.add(1);
int size = queue.size();
for(int i=0; i<size; i++) {
    System.out.println("poll "+i+": "+queue.poll());
}

queue.peek();  //peek값 삭제 안하고 출력
queue.poll();  //삭제

//min heap : 1,2,3,7
//max heap : 7,3,2,1