Top K Frequent Elements

/*
 * Problem : Process an array and a number k.
 * Input   : n, integers, then k.
 * Output  : The k most frequent values.
 * Approach: Count, sort, take k.
 * Time    : O(n + d log d)
 * Space   : O(d)
 * Java    : 8 or newer.
 */
import java.util.Scanner;
import java.util.Arrays;
import java.util.stream.Collectors;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static int[] solve(int[] a, int k) {
        // Step 1: Count how often each value appears.
        java.util.Map<Integer, Integer> counts = new java.util.HashMap<>();
        for (int value : a) {
            counts.merge(value, 1, Integer::sum);
        }

        // Step 2: Put the different values in a list and sort it. A higher count goes first. Equal counts: smaller value first.
        java.util.List<Integer> values = new java.util.ArrayList<>(counts.keySet());
        values.sort((x, y) -> {
            int byCount = counts.get(y) - counts.get(x);
            return byCount != 0 ? byCount : Integer.compare(x, y);
        });

        // Step 3: Take the first k values.
        int[] result = new int[k];
        for (int i = 0; i < k; i++) {
            result[i] = values.get(i);
        }
        return result;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        int k = scanner.nextInt();
        System.out.println(Arrays.stream(solve(a, k)).mapToObj(String::valueOf).collect(Collectors.joining(" ")));
    }
}