347. Top K Frequent Elements

用了新的结构~priorityQueue，记得再看看用法哦

    public List<Integer> topKFrequent(int[] nums, int k) {
        List<Integer> res = new ArrayList<Integer>();
        if(nums.length == 0) {
            return res;
        }
        PriorityQueue<Entry<Integer, Integer>> pq = new PriorityQueue<Entry<Integer, Integer>>(
            new Comparator<Entry<Integer, Integer>>() {
                public int compare(Entry<Integer, Integer> o1, Entry<Integer, Integer> o2) {
                    return o1.getValue() - o2.getValue();
                }
            });
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for(int i = 0; i < nums.length; i++) {
            if(map.containsKey(nums[i])) {
                map.put(nums[i], map.get(nums[i]) + 1);
            } else {
                map.put(nums[i], 1);
            }
        }
        for(Entry<Integer, Integer> entry: map.entrySet()) {
            if(pq.size() < k) {
                pq.add(entry);
            } else {
                if(pq.peek().getValue() < entry.getValue()) {
                    pq.poll();
                    pq.add(entry);
                }
            }
        }
        while(!pq.isEmpty()) {
            Entry<Integer, Integer> entry = pq.poll();
            res.add(0, entry.getKey());
        }
        return res;
    }