Leetcode no. 347

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) {
List<Integer> res= new LinkedList<>();
List<Integer>[] buckets= new List[nums.length+1];
Map<Integer, Integer> map= new HashMap<>();
for (int ele: nums) {
map.put(ele, map.getOrDefault(ele, 0)+1);
}
for (int key: map.keySet()) {
int fre= map.get(key);
if (buckets[fre]== null) buckets[fre]= new LinkedList<>();
buckets[fre].add(key);
}
for (int i = buckets.length-1; i >= 0 && res.size()<k; i--) {
if (buckets[i]!= null) res.addAll(buckets[i]);
}
return res;
}
}