Reversing single linked list in C#

Question

I am trying to reverse a linked list. This is the code I have come up with:

 public static void Reverse(ref Node root)
 {
      Node tmp = root;
      Node n         


        

Answer 1

linked list reversal recursive

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ReverseLinkedList
{
    class Program
    {
        static void Main(string[] args)
        {
            Node head = null;
            LinkedList.Append(ref head, 25);
            LinkedList.Append(ref head, 5);
            LinkedList.Append(ref head, 18);
            LinkedList.Append(ref head, 7);

            Console.WriteLine("Linked list:");
            LinkedList.Print(head);

            Console.WriteLine();
            Console.WriteLine("Reversed Linked list:");
            LinkedList.Reverse(ref head);
            LinkedList.Print(head);

            Console.WriteLine();
            Console.WriteLine("Reverse of Reversed Linked list:");
            LinkedList.ReverseUsingRecursion(head);
            head = LinkedList.newHead;
            LinkedList.PrintRecursive(head);
        }

        public static class LinkedList
        {
            public static void Append(ref Node head, int data)
            {
                if (head != null)
                {
                    Node current = head;
                    while (current.Next != null)
                    {
                        current = current.Next;
                    }

                    current.Next = new Node();
                    current.Next.Data = data;
                }
                else
                {
                    head = new Node();
                    head.Data = data;
                }
            }

            public static void Print(Node head)
            {
                if (head == null) return;
                Node current = head;
                do
                {
                    Console.Write("{0} ", current.Data);
                    current = current.Next;
                } while (current != null);
            }

            public static void PrintRecursive(Node head)
            {
                if (head == null)
                {
                    Console.WriteLine();
                    return;
                }
                Console.Write("{0} ", head.Data);
                PrintRecursive(head.Next);
            }

            public static void Reverse(ref Node head)
            {
                if (head == null) return;
                Node prev = null, current = head, next = null;
                while (current.Next != null)
                {
                    next = current.Next;
                    current.Next = prev;
                    prev = current;
                    current = next;
                }
                current.Next = prev;
                head = current;
            }

            public static Node newHead;

            public static void ReverseUsingRecursion(Node head)
            {
                if (head == null) return;
                if (head.Next == null)
                {
                    newHead = head;
                    return;
                }

                ReverseUsingRecursion(head.Next);
                head.Next.Next = head;
                head.Next = null;
            }
        }

        public class Node
        {
            public int Data = 0;
            public Node Next = null;
        }
    }
}

Answer 2

Node p = root, n = null;
while (p != null) {
    Node tmp = p.next;
    p.next = n;
    n = p;
    p = tmp;
}
root = n;

Answer 3

Here a sample code to reverse a linked list.

using System;

class Program
{
    static void Main(string[] args)
    {
        LinkItem item = generateLinkList(5);
        printLinkList(item);
        Console.WriteLine("Reversing the list ...");
        LinkItem newItem = reverseLinkList(item);
        printLinkList(newItem);
        Console.ReadLine();
    }

    static public LinkItem generateLinkList(int total)
    {
        LinkItem item = new LinkItem();
        for (int number = total; number >=1; number--)
        {
            item = new LinkItem
            {
                name = string.Format("I am the link item number {0}.", number),
                next = (number == total) ? null : item
            };
        }
        return item;
    }

    static public void printLinkList(LinkItem item)
    {
        while (item != null)
        {
            Console.WriteLine(item.name);
            item = item.next;
        }
    }

    static public LinkItem reverseLinkList(LinkItem item)
    {
        LinkItem newItem = new LinkItem
        {
            name = item.name,
            next = null
        };

        while (item.next != null)
        {
            newItem = new LinkItem
            {
                name = item.next.name,
                next = newItem
            };

            item = item.next;
        }

        return newItem;
    }
}

class LinkItem
{
    public string name;
    public LinkItem next;
}

Answer 4

You don't need to make a copy. Some pseudo code:

prev = null;
current = head;
next = current->next;

(while next != null)
    current->next=prev
    prev=current
    current=next
    next=current->next

Answer 5

This performed pretty well on Leetcode.

public ListNode ReverseList(ListNode head) {

        ListNode previous = null;
        ListNode current = head; 
        while(current != null) {
            ListNode nextTemp = current.next;
            current.next = previous;
            previous = current;
            current = nextTemp;
        }

        return previous;
    }     

Answer 6

Years ago I missed out on a hipster-L.A.-entertainment-company ASP.NET MVC developer position because I could not answer this question :( (It's a way to weed out non-computer-science majors.) So I am embarrassed to admit that it took me way too long to figure this out in LINQpad using the actual LinkedList<T>:

var linkedList = new LinkedList<int>(new[]{1,2,3,4,5,6,7,8,9,10});
linkedList.Dump("initial state");

var head = linkedList.First;
while (head.Next != null)
{
    var next = head.Next;
    linkedList.Remove(next);
    linkedList.AddFirst(next.Value);
}

linkedList.Dump("final state");

The read-only LinkedListNode<T>.Next property is what makes LinkedList<T> so important here. (Non-comp-sci people are encouraged to study the history of Data Structures---we are supposed to ask the question, Where does the linked list come from---why does it exist?)