Problem

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

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.

Solution

public class Solution {
    public List<Integer> topKFrequent(int[] nums, int k) {
        int n = nums.length;
        Map<Integer, Integer> map = new HashMap<>();
        for (int num: nums) {
            if (map.containsKey(num)) map.put(num, map.get(num)+1);
            else map.put(num, 1);
        }
        List<Integer>[] freq = new ArrayList[n+1];
        for (int num : map.keySet()) {
            int i = map.get(num);
            if (freq[i] == null) {
                freq[i] = new ArrayList<>();
            }
            freq[i].add(num);
        }
        List<Integer> res = new ArrayList<>();
        for (int index = freq.length - 1; index >= 0 && res.size() < k; index--) {
            if (freq[index] != null) {
                res.addAll(freq[index]);
            }
        }
        return res;
    }
}