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]
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 List<Integer> topKFrequent(int[] nums, int k) {
List<Integer>[] bucket = new ArrayList[nums.length + 1];
HashMap<Integer,Integer> fMap = new HashMap<Integer,Integer>();
for(int n : nums){
fMap.put(n, fMap.getOrDefault(n, 0) + 1);
}
for(int key : fMap.keySet()){
int frequency = fMap.get(key);
if (bucket[frequency] == null) {
bucket[frequency] = new ArrayList<>();
}
bucket[frequency].add(key);
}
List<Integer> res = new ArrayList<>();
for (int pos = bucket.length - 1; pos >= 0 && res.size() < k; pos--) {
if (bucket[pos] != null) {
res.addAll(bucket[pos]);
}
}
return res;
}