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.

void reverseList(struct Node** head_ref) {
  //1. If head is not null create three nodes
  //   prevNode - pointing to head,
  //   tempNode - pointing to head,
  //   curNode - pointing to next of head
  if(*head_ref != NULL) {
    struct Node* prevNode = *head_ref;
    struct Node* tempNode = *head_ref;
    struct Node* curNode = (*head_ref)->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 != *head_ref) {
      //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;
      (*head_ref)->next = curNode;
      curNode->prev = *head_ref;

      prevNode = curNode;
      curNode = tempNode;
    }

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

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

#include <stdio.h>
#include <stdlib.h>

//node structure
struct Node {
  int data;
  struct Node* next;
  struct Node* prev;
};

//Add new element at the end of the list
void push_back(struct Node** head_ref, int newElement) {  
  struct Node *newNode, *temp;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;  
  newNode->next = NULL;
  newNode->prev = NULL;
  if(*head_ref == NULL) {
    *head_ref = newNode;
     newNode->next = *head_ref;
     newNode->prev = *head_ref;
  } else {
    temp = *head_ref;
    while(temp->next != *head_ref) {
      temp = temp->next;
    }    
    temp->next = newNode;
    newNode->next = *head_ref;
    newNode->prev = temp;
    (*head_ref)->prev = newNode;
  }
}

//reverse the list
void reverseList(struct Node** head_ref) {
  if(*head_ref != NULL) {
    struct Node* prevNode = *head_ref;
    struct Node* tempNode = *head_ref;
    struct Node* curNode = (*head_ref)->next;
    
    prevNode->next = prevNode;
    prevNode->prev = prevNode;
    
    while(curNode != *head_ref) { 
      tempNode = curNode->next;

      curNode->next = prevNode;
      prevNode->prev = curNode;
      (*head_ref)->next = curNode;
      curNode->prev = *head_ref;

      prevNode = curNode;
      curNode = tempNode;
    }

    *head_ref = prevNode;
  }
} 

//display the content of the list
void PrintList(struct Node* head_ref) {
  struct Node* temp = head_ref;
  if(head_ref != NULL) {
    printf("The list contains: ");
    while (1) {
      printf("%i ",temp->data);
      temp = temp->next;
      if(temp == head_ref)
        break;    
    }
    printf("\n");
  } else {
    printf("The list is empty.\n");
  }   
}

// test the code 
int main() {
  struct Node* MyList = NULL;

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

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

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

  return 0; 
}

The above code will give the following output:

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