C - Delete odd nodes of the Doubly Linked List

Deleting odd nodes of a doubly linked list requires traverse through the list and deleting odd nodes one by one. If the list is not null then release the head and make next of head as new head. If the new head is not null then make the next of head as null and create two nodes - evenNode and oddNode. Make evenNode to the first even node of list and oddNode to second odd node of the list (3^rd node in the original list). If both are not null, delete the oddNode and adjust links. Move both nodes to next set of even-odd nodes. Repeat the process till the any or both nodes become null.

The function deleteOddNodes is created for this purpose. It is a 5-step process.

void deleteOddNodes(struct Node** head_ref) {
  if(*head_ref != NULL) {
    
    //1. if head is not null, make next of head as
    //   new head and delete previous head
    struct Node* temp = *head_ref;
    *head_ref = (*head_ref)->next;
    free(temp);
    if(*head_ref != NULL) {
      
      //2. if the new head is not make prev of head as
      //   null and create nodes - evenNode and oddNode
      (*head_ref)->prev = NULL;
      struct Node* evenNode = *head_ref;
      struct Node* oddNode = (*head_ref)->next;

      while(evenNode != NULL && oddNode != NULL) {
        
        //3. while evenNode and oddNode are not null
        //   make next of evenNode as next of oddNode 
        //   and free oddNode   
        evenNode->next = oddNode->next;
        free(oddNode);

        //4. and make temp as evenNode and evenNode as 
        //   next of evenNode
        temp = evenNode;
        evenNode = evenNode->next;
        
        //5. Update prev link, evenNode and oddNode
        if(evenNode != NULL) {
          evenNode->prev = temp;
          oddNode = evenNode->next;
        }
      }
    }
  }   
}

The below is a complete program that uses above discussed concept of deleting odd nodes of a 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;
  } else {
    temp = *head_ref;
    while(temp->next != NULL) {
      temp = temp->next;
    }    
    temp->next = newNode;
    newNode->prev = temp;
  }
}

//delete odd nodes of the list
void deleteOddNodes(struct Node** head_ref) {
  if(*head_ref != NULL) {
    struct Node* temp = *head_ref;
    *head_ref = (*head_ref)->next;
    free(temp);
    if(*head_ref != NULL) {
      (*head_ref)->prev = NULL;
      struct Node* evenNode = *head_ref;
      struct Node* oddNode = (*head_ref)->next;
      while(evenNode != NULL && oddNode != NULL) {
        evenNode->next = oddNode->next;
        free(oddNode);
        temp = evenNode;
        evenNode = evenNode->next;
        if(evenNode != NULL) {
          evenNode->prev = temp;
          oddNode = evenNode->next;
        }
      }
    }
  }   
}    

//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 (temp != NULL) {
      printf("%i ",temp->data);
      temp = temp->next;  
    }
    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);

  //delete odd nodes of the list
  deleteOddNodes(&MyList);

  printf("After deleting odd nodes.\n");
  //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 
After deleting odd nodes.
The list contains: 20 40