Java中关于优先队列PriorityQueue的使用及相关方法

PriorityQueue<Integer> p = new PriorityQueue<>();

PriorityQueue<Integer> p = new PriorityQueue<>();
p.offer(5);
p.offer(1);
p.offer(3);
p.offer(6);
p.offer(8);
while(!p.isEmpty()) {
	System.out.println(p.poll());
}

PriorityQueue<Integer> queue=new PriorityQueue<>((o1,o2)->o2-o1);
PriorityQueue<Integer> queue= new PriorityQueue<>(Comparator.reverseOrder());

public static void main(String[] args) {
	HashMap<Integer, Integer> map = new HashMap<>();
	map.put(1, 2);
	map.put(3, 4);
	map.put(5, 6);
	Set<Map.Entry<Integer, Integer>> entries = map.entrySet();//获取键值对
	//自定义优先队列的排序规则，队列内存放的数据类型是键值对
	//o1，o2都是键值对这样的对象
	PriorityQueue<Map.Entry<Integer, Integer>> queue = new PriorityQueue<>((o1, o2) -> {
		return o1.getValue() - o2.getValue();//根据值升序排序
	});
	for (Map.Entry<Integer, Integer> entry : entries) {
		queue.offer(entry);//将键值对放到优先队列中
    }
	while(!queue.isEmpty()) {
		System.out.println(queue.poll().getValue());
	}
}

public static void main(String[] args) {
        PriorityQueue<Integer> queue = new PriorityQueue<>();
        queue.offer(1);
        queue.offer(2);
        queue.offer(3);
        queue.offer(4);
        queue.offer(5);
        queue.offer(6);
        while (!queue.isEmpty()){
            System.out.print(queue.poll() + " ");
        }
    }

public static void main(String[] args) {
  PriorityQueue<Integer> queue = new PriorityQueue<>((o1,o2) -> o2 - o1);//降序
        queue.offer(1);
        queue.offer(2);
        queue.offer(3);
        queue.offer(4);
        queue.offer(5);
        queue.offer(6);
        while (!queue.isEmpty()){
            System.out.print(queue.poll() + " ");
        }
    }

public int[] topKFrequent(int[] nums, int k) {
        //key为元素值，value为出现频率
        HashMap<Integer,Integer> map = new HashMap<>();
        PriorityQueue<Map.Entry<Integer, Integer>> queue 
        = new PriorityQueue<>((o1,o2)-> o1.getValue() - o2.getValue());
        int[] res = new int[k];
        for (int num : nums) {
            map.put(num,map.getOrDefault(num,0) + 1);
        }
        Set<Map.Entry<Integer, Integer>> entries = map.entrySet();
        for (Map.Entry<Integer, Integer> entry : entries) {
            queue.offer(entry);
            if(queue.size() > k){
                queue.poll();
            }
        }
        //留下的都是出现频率最高的
        for(int i = 0; i < k; i++){
            Map.Entry<Integer, Integer> poll = queue.poll();
            res[i] = poll.getKey();
        }
        return res;

public List<String> topKFrequent(String[] words, int k) {
        List<String> res = new ArrayList<>();
        //key为元素，value为出现频率
        HashMap<String,Integer> map = new HashMap<>();
        for (String word : words) {
            map.put(word,map.getOrDefault(word,0) + 1);
        }
        PriorityQueue<Map.Entry<String,Integer>> queue = new PriorityQueue<>(
                ((o1, o2) -> {
                    if(o1.getValue() == o2.getValue()){
                        return o2.getKey().compareTo(o1.getKey());
                    }
                    return o1.getValue() - o2.getValue();
                })
        );
        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        for (Map.Entry<String, Integer> entry : entries) {
            queue.offer(entry);
            if(queue.size() > k){
                queue.poll();
            }
        }
        for(int i = 0; i < k; i++){
            res.add(queue.poll().getKey());
        }
        Collections.reverse(res);
        return res;
    }