前k个高频元素

输入：arr={2, 2, 2, 2, 3, 3, 3, 1}，k = 2;

输出：[3,2];

利用PrioriQueue的特性和Map的数据结构，返回前k个高频元素。

public static void main(String[] args) {
        List<Integer> firstKNums = getFirstKNums(new int[]{2, 2, 2, 2, 3, 3, 3, 1}, 2);
        System.out.println(firstKNums);
    }
 
    public static List<Integer> getFirstKNums(int[] arr, int k) {
        Map<Integer, Integer> map = Maps.newHashMap();
        for (int i = 0; i < arr.length; i++) {
            if (map.containsKey(arr[i])) {
                map.put(arr[i], map.get(arr[i]) + 1);
            } else {
                map.put(arr[i], 1);
            }
        }
        LinkedList<Integer> link = new LinkedList<>();
 
        PriorityQueue priorityQueue = new PriorityQueue<Map.Entry>((entry1,entry2)->
             Integer.valueOf(entry1.getValue().toString())-Integer.parseInt(entry2.getValue().toString())
        );
        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
            priorityQueue.offer(entry);
        }
        if (priorityQueue.size()>k) {
            priorityQueue.poll();
        }
        for (int i = k; i >0; i--) {
            link.add(((Map.Entry<Integer, Integer>)priorityQueue.poll()).getKey());
        }
        return link;
    }