C# - Reverse the Circular Doubly Linked List

While working with a circular doubly linked list, sometimes it is required to reverse it. Reversing a List produces following result: if the given List is 10->20->30->40->50, after reversing the List the List becomes 50->40->30->20->10.

Reversing a List requires creating three nodes, considering that the list is not empty, which are as follows: tempNode pointing to head, prevNode pointing to head and curNode pointing to next of head. Then make next and previous of prevNode as itself to make the first node as last node of the reversed list. After that, while the curNode is not head adjust links (unlink curNode and link it to the reversed list from front and modify curNode and prevNode to the next element in the list). At last, make the prevNode (last node) as head.

The function reverseList is created for this purpose. It is a 4-step process.

public void reverseList() {
  //1. If head is not null create three nodes
  //   prevNode - pointing to head,
  //   tempNode - pointing to head,
  //   curNode - pointing to next of head
  if(this.head != null) {
    Node prevNode = this.head;
    Node tempNode = this.head;
    Node curNode = this.head.next;
    
    //2. assign next and previous of prevNode 
    //   as itself to make the first node as last 
    //   node of the reversed list
    prevNode.next = prevNode;
    prevNode.prev = prevNode;
    
    while(curNode != this.head) {
      //3. While the curNode is not head adjust links 
      //   (unlink curNode and link it to the reversed list 
      //   from front and modify curNode and prevNode) 
      tempNode = curNode.next;

      curNode.next = prevNode;
      prevNode.prev = curNode;
      this.head.next = curNode;
      curNode.prev = this.head;

      prevNode = curNode;
      curNode = tempNode;
    }

    //4. Make prevNode (last node) as head
    this.head = prevNode;
  }
}

The below is a complete program that uses above discussed concept to reverse a given circular doubly linked list.

using System;

//node structure
class Node {
  public int data;
  public Node next;
  public Node prev;
};

class LinkedList {
  Node head;

  public LinkedList(){
    head = null;
  }
  
  //Add new element at the end of the list
  public void push_back(int newElement) {
    Node newNode = new Node();
    newNode.data = newElement;
    newNode.next = null; 
    newNode.prev = null;
    if(head == null) {
      head = newNode;
      newNode.next = head;
      newNode.prev = head;
    } else {
      Node temp = new Node();
      temp = head;
      while(temp.next != head)
        temp = temp.next;
      temp.next = newNode;
      newNode.next = head;
      newNode.prev = temp;
      head.prev = newNode;
    }    
  }

  //reverse the list
  public void reverseList() {
    if(this.head != null) {
      Node prevNode = this.head;
      Node tempNode = this.head;
      Node curNode = this.head.next;
      
      prevNode.next = prevNode;
      prevNode.prev = prevNode;
      
      while(curNode != this.head) {
        tempNode = curNode.next;

        curNode.next = prevNode;
        prevNode.prev = curNode;
        this.head.next = curNode;
        curNode.prev = this.head;

        prevNode = curNode;
        curNode = tempNode;
      }

      this.head = prevNode;
    }
  }  

  //display the content of the list
  public void PrintList() {
    Node temp = new Node();
    temp = this.head;
    if(temp != null) {
      Console.Write("The list contains: ");
      while(true) {
        Console.Write(temp.data + " ");
        temp = temp.next;
        if(temp == this.head)
          break;        
      }
      Console.WriteLine();
    } else {
      Console.WriteLine("The list is empty.");
    }
  }      
};

// test the code
class Implementation {  
  static void Main(string[] args) {
    LinkedList MyList = new LinkedList();

    //Add five elements in the list.
    MyList.push_back(10);
    MyList.push_back(20);
    MyList.push_back(30);
    MyList.push_back(40);
    MyList.push_back(50);

    //Display the content of the list.
    MyList.PrintList();  

    //Reversing the list.
    MyList.reverseList();
  
    //Display the content of the list.
    MyList.PrintList();  
  }
}

The above code will give the following output:

The list contains: 10 20 30 40 50 
The list contains: 50 40 30 20 10