C - Insert a new node at the start of the Circular Doubly Linked List

In this method, a new node is inserted at the start of the circular doubly linked list. For example - if the given List is 10->20->30 and a new element 100 is added at the start, the List becomes 100->10->20->30.

Inserting a new node at the start of the circular doubly linked list is very easy. First, a new node with given element is created. It is then added at the start of the list by linking the head node and last node to the new node.

Circular Doubly Linked List - Add Node At Start

The function push_front is created for this purpose. It is a 6-step process.

void push_front(struct Node** head_ref, int newElement) {  
  
  //1. allocate node
  struct Node *newNode, *temp;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  
  //2. assign data element
  newNode->data = newElement;  
  
  //3. assign null to the next and prev
  //   of the new node
  newNode->next = NULL;
  newNode->prev = NULL;

  //4. Check the list is empty or not,
  //   if empty make the new node as head 
  if(*head_ref == NULL) {
    *head_ref = newNode;
    newNode->next = *head_ref;
    newNode->prev = *head_ref;
  } else {
    
    //5. Else, traverse to the last node
    temp = *head_ref;
    while(temp->next != *head_ref) {
      temp = temp->next;
    }    
    
    //6. Adjust the links
    temp->next = newNode;
    newNode->prev = temp;
    newNode->next = *head_ref;
    (*head_ref)->prev = newNode;
    *head_ref = newNode;
  }
}

The below is a complete program that uses above discussed concept to insert new node at the start of the 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 start of the list
void push_front(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->prev = temp;
    newNode->next = *head_ref;
    (*head_ref)->prev = newNode;
    *head_ref = newNode;
  }
}

//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 three elements at the start of the list.
  push_front(&MyList, 10);
  push_front(&MyList, 20);
  push_front(&MyList, 30);
  PrintList(MyList);

  return 0; 
}

The above code will give the following output:

The list contains: 30 20 10