C - Delete the last node of the Circular Doubly Linked List

In this method, the last node of the circular doubly linked list is deleted. For example - if the given list is 10->20->30->40 and the last node is deleted, the list becomes 10->20->30.

Deleting the last node of the Circular Doubly Linked List involves checking the head for empty. If it is not empty and contains only one node then delete the head node. If the list contains more than one node then traverse to the second last node of the list and link it with the head. Finally, delete the last node.

Circular Doubly Linked List - Delete Last Node

The function pop_back is created for this purpose. It is a 3-step process.

void pop_back(struct Node** head_ref) {
  if(*head_ref != NULL) {
    
    //1. if head in not null and next of head
    //   is head, release the head
    if((*head_ref)->next == *head_ref) {
      *head_ref = NULL;
    } else {
      
      //2. Else, traverse to the second last 
      //   element of the list
      struct Node* temp = *head_ref;
      while(temp->next->next != *head_ref)
        temp = temp->next;
      
      //3. Update links of head and second 
      //   last node, and delete the last node     
      struct Node* lastNode = temp->next;
      temp->next = *head_ref;
      (*head_ref)->prev = temp;
      free(lastNode); 
    }
  }
}

The below is a complete program that uses above discussed concept of deleting the last node 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 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;
  }
}

//Delete last node of the list
void pop_back(struct Node** head_ref) {
  if(*head_ref != NULL) {
    if((*head_ref)->next == *head_ref) {
      *head_ref = NULL;
    } else {
      struct Node* temp = *head_ref;
      while(temp->next->next != *head_ref)
        temp = temp->next;   
      struct Node* lastNode = temp->next;
      temp->next = *head_ref;
      (*head_ref)->prev = temp;
      free(lastNode); 
    }
  }
}

//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 four elements in the list.
  push_back(&MyList, 10);
  push_back(&MyList, 20);
  push_back(&MyList, 30);
  push_back(&MyList, 40);
  PrintList(MyList);

  //Delete the last node
  pop_back(&MyList);
  PrintList(MyList);

  return 0; 
}

The above code will give the following output:

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