Parallel Merge Sort (Fork/Join)

/*
 * Problem : Sorted array.
 * Input   : n, then n integers.
 * Output  : The sorted array.
 * Approach: Fork, sort, join, merge.
 * Time    : O(n log n) work
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;
import java.util.Arrays;
import java.util.stream.Collectors;

public class Main {
        // Step 1: A RecursiveTask returns a result. Each task sorts one part of the array.
        static class SortTask extends java.util.concurrent.RecursiveTask<int[]> {
            private final int[] part;

            SortTask(int[] part) {
                this.part = part;
            }

            @Override
            protected int[] compute() {
                // Step 2: Small parts are sorted directly. Forking tiny parts costs more than it saves.
                if (part.length <= 8) {
                    int[] copy = part.clone();
                    java.util.Arrays.sort(copy);
                    return copy;
                }
                // Step 3: Fork the left half to another worker, sort the right half here, then wait for the left half.
                int middle = part.length / 2;
                SortTask left = new SortTask(java.util.Arrays.copyOfRange(part, 0, middle));
                SortTask right = new SortTask(java.util.Arrays.copyOfRange(part, middle, part.length));
                left.fork();
                int[] sortedRight = right.compute();
                int[] sortedLeft = left.join();
                return merge(sortedLeft, sortedRight);
            }
        }

        // Step 4: Merge two sorted arrays: always take the smaller front element.
        static int[] merge(int[] x, int[] y) {
            int[] result = new int[x.length + y.length];
            int i = 0;
            int j = 0;
            int k = 0;
            while (i < x.length && j < y.length) {
                result[k++] = x[i] <= y[j] ? x[i++] : y[j++];
            }
            while (i < x.length) {
                result[k++] = x[i++];
            }
            while (j < y.length) {
                result[k++] = y[j++];
            }
            return result;
        }

    // This method solves the task. It receives the input and returns the answer.
    static int[] solve(int[] a) {
        return java.util.concurrent.ForkJoinPool.commonPool().invoke(new SortTask(a));
    }

    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();
        }
        System.out.println(Arrays.stream(solve(a)).mapToObj(String::valueOf).collect(Collectors.joining(" ")));
    }
}