TopKFrequentElements.java

import java.util.*;

public class TopKFrequentElements {

    public static int[] topKFrequent(int[] nums, int k) {
        // Step 1: Count frequencies
        Map<Integer, Integer> freqMap = new HashMap<>();
        for (int num : nums) {
            freqMap.put(num, freqMap.getOrDefault(num, 0) + 1);
        }

        // Step 2: Use a min-heap to keep top k elements
        PriorityQueue<Map.Entry<Integer, Integer>> minHeap =
                new PriorityQueue<>((a, b) -> a.getValue() - b.getValue());

        for (Map.Entry<Integer, Integer> entry : freqMap.entrySet()) {
            minHeap.offer(entry);
            if (minHeap.size() > k) {
                minHeap.poll(); // remove least frequent element
            }
        }

        // Step 3: Extract elements from heap
        int[] result = new int[k];
        for (int i = k - 1; i >= 0; i--) {
            result[i] = minHeap.poll().getKey();
        }

        return result;
    }

    public static void main(String[] args) {
        int[] nums = {1, 1, 1, 2, 2, 3, 3, 3, 3, 4};
        int k = 2;

        int[] res = topKFrequent(nums, k);
        System.out.println("Top " + k + " frequent elements: " + Arrays.toString(res));
    }
}