347. Top K Frequent Elements

Given a non-empty array of integers, return the k most frequent elements.

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note: 

• You may assume k is always valid, 1 ≤ k ≤ number of unique elements.
• Your algorithm's time complexity must be better than O(n log n), where n is the array's size.

题目含义：给出一个非空整数列表，返回出现频次最大的k个元素

    public List<Integer> topKFrequent(int[] nums, int k) {
        List<Integer> result = new ArrayList<>();
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            map.put(nums[i], map.getOrDefault(nums[i], 0)+1);
        }
        if (map.size() < k) return result;
        PriorityQueue<Map.Entry<Integer, Integer>> pq = new PriorityQueue<>(
                new Comparator<Map.Entry<Integer, Integer>>() {
                    @Override
                    public int compare(Map.Entry<Integer, Integer> a, Map.Entry<Integer, Integer> b) {
                        return b.getValue() - a.getValue();
                    }
                }
        );
        pq.addAll(map.entrySet());
        for (int i = 0; i < k; i++) {
            result.add(pq.poll().getKey());
        }
        return result;       
    }