Merge Two Sorted Lists

/*
 * Problem : Merge two sorted lists.
 * Input   : Two counts and two arrays.
 * Output  : The merged list.
 * Approach: Dummy head node.
 * Time    : O(n + m)
 * Space   : O(1) extra
 * Java    : 8 or newer.
 */
import java.util.Scanner;
import java.util.Arrays;
import java.util.stream.Collectors;

public class Main {
        // Step 1: A node holds one value and a link to the next node. The last node links to null.
        static class Node {
            int value;
            Node next;

            Node(int value) {
                this.value = value;
            }
        }

        // This method builds a linked list from an array and returns the first node.
        static Node build(int[] values) {
            Node head = null;
            Node tail = null;
            for (int value : values) {
                Node node = new Node(value);
                if (head == null) {
                    head = node;
                } else {
                    tail.next = node;
                }
                tail = node;
            }
            return head;
        }

    // This method solves the task. It receives the input and returns the answer.
    static int[] solve(int[] a, int[] b) {
        // Step 2: Build two linked lists.
        Node headA = build(a);
        Node headB = build(b);

        // Step 3: dummy is a helper node in front of the result. This avoids special code for the first node.
        Node dummy = new Node(0);
        Node tail = dummy;

        // Step 4: Always take the smaller front node of the two lists. Use <= so that equal values keep their order.
        while (headA != null && headB != null) {
            if (headA.value <= headB.value) {
                tail.next = headA;
                headA = headA.next;
            } else {
                tail.next = headB;
                headB = headB.next;
            }
            tail = tail.next;
        }

        // Step 5: One list is empty now. Link the rest of the other list.
        tail.next = (headA != null) ? headA : headB;

        // Step 6: Copy the values to an array for printing.
        int[] result = new int[a.length + b.length];
        int index = 0;
        for (Node node = dummy.next; node != null; node = node.next) {
            result[index++] = node.value;
        }
        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 sizeB = scanner.nextInt();
        int[] b = new int[sizeB];
        for (int i = 0; i < sizeB; i++) {
            b[i] = scanner.nextInt();
        }
        System.out.println(Arrays.stream(solve(a, b)).mapToObj(String::valueOf).collect(Collectors.joining(" ")));
    }
}