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.




public class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {
        HashMap<Integer,Integer> map = new HashMap<>();
        ArrayList<Integer> result = new ArrayList<>();
        for (int i = 0; i < nums.length; i++) map.put(nums[i],map.getOrDefault(nums[i],0)+1);
        ArrayList<Integer> alkey = new ArrayList<>(map.keySet());
        Collections.sort(alkey, (o1, o2) -> map.get(o2)-map.get(o1));
        for(int i = 0;i<k;i++) result.add(alkey.get(i));
        return result;