Reverse a Linked List

/*
 * Problem : Process the list.
 * Input   : n, then n integers.
 * Output  : The reversed list.
 * Approach: Turn every link around.
 * Time    : O(n)
 * 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 solves the task. It receives the input and returns the answer.
    static int[] solve(int[] a) {
        // Step 2: Build the list. head is the first node, tail is the last node.
        Node head = null;
        Node tail = null;
        for (int value : a) {
            Node node = new Node(value);
            if (head == null) {
                head = node;
            } else {
                tail.next = node;
            }
            tail = node;
        }

        // Step 3: Reverse. previous is the already reversed part. current is the node we handle now.
        Node previous = null;
        Node current = head;
        while (current != null) {
            // Step 4: Remember the next node, turn the link back, and move both pointers one step.
            Node next = current.next;
            current.next = previous;
            previous = current;
            current = next;
        }

        // Step 5: previous is the new head. Copy the values to an array for printing.
        int[] result = new int[a.length];
        int index = 0;
        for (Node node = previous; 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();
        }
        System.out.println(Arrays.stream(solve(a)).mapToObj(String::valueOf).collect(Collectors.joining(" ")));
    }
}