347. 前 K 个高频元素

/**
 * 就是用优先队列解决问题，特别经典的一个case
 * 值得深入记忆
 */
class Solution {
    public int[] topKFrequent(int[] nums, int k) {
        Map<Integer,Integer> map = new HashMap<>();
        for(Integer i  : nums){
            map.put(i,map.getOrDefault(i,0) + 1);
        }
        PriorityQueue<Integer[]> priorityQueue = new PriorityQueue<Integer[]>(k, new Comparator<Integer[]>() {
            @Override
            public int compare(Integer[] o1, Integer[] o2) {
                Integer num1 = o1[1];
                Integer num2 = o2[1];
                return num1.compareTo(num2);
            }
        });
        for(Map.Entry<Integer,Integer> entry : map.entrySet()){
            Integer key = entry.getKey();
            Integer num = entry.getValue();
            if(priorityQueue.size() == k){
                if(priorityQueue.peek()[1] < num){
                    priorityQueue.poll();
                    priorityQueue.add(new Integer[]{key,num});
                }
            }else {
                priorityQueue.add(new Integer[]{key,num});
            }
        }
        int[] ret = new int[k];
        for (int i = 0; i < k; ++i) {
            ret[i] = priorityQueue.poll()[0];
        }
        return ret;
    }
}