347. Top K Frequent Elements

Question

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

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

Example 2:

Input: nums = [1], k = 1
Output: [1]

Thinking:

• Method 1:

class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<>();
        List<Integer> res = new ArrayList<>();
        for(int num : nums){
            map.put(num, map.containsKey(num)?map.get(num) + 1: 1);
        }
        Set<Map.Entry<Integer, Integer>> set = map.entrySet();
        PriorityQueue<Map.Entry<Integer, Integer>> pq = new PriorityQueue<>(new Comparator<Map.Entry<Integer, Integer>>(){
            @Override
            public int compare(Map.Entry<Integer, Integer> n1, Map.Entry<Integer, Integer> n2){
                return n2.getValue() - n1.getValue();
            }
        });
        for(Map.Entry<Integer, Integer> entry: set){
            pq.offer(entry);
        }
        for(int i = 0; i < k; i++){
            res.add(pq.poll().getKey());
        }
        return res;
    }
}