Delete all the odd nodes from a Doubly Linked List

Given a doubly linked list containing N nodes, the task is to delete all the odd nodes from the list.

Examples :

Input: Initial List = 14  15  5  6  16
Output: Final List = 15  5 

Input: Initial List = 6  4  7  9  5  2  3  10
Output: Final List =  7 4  5  3

The idea is to traverse the nodes of the doubly linked list one by one and get the pointer of the nodes having data odd. Delete those nodes.

Below is the implementation of above idea:

// C++ implementation to delete all 
// the odd nodes from the doubly 
// linked list 
#include <bits/stdc++.h> 

using namespace std; 

// Node of the doubly linked list 
struct Node { 
	int data; 
	Node *prev, *next; 
}; 

// function to insert a node at the beginning 
// of the Doubly Linked List 
void push(Node** head_ref, int new_data) 
{ 
	// allocate node 
	Node* new_node = (Node*)malloc(sizeof(struct Node)); 

	// put in the data 
	new_node->data = new_data; 

	// since we are adding at the begining, 
	// prev is always NULL 
	new_node->prev = NULL; 

	// link the old list off the new node 
	new_node->next = (*head_ref); 

	// change prev of head node to new node 
	if ((*head_ref) != NULL) 
		(*head_ref)->prev = new_node; 

	// move the head to point to the new node 
	(*head_ref) = new_node; 
} 

// function to delete a node in a Doubly Linked List. 
// head_ref --> pointer to head node pointer. 
// del --> pointer to node to be deleted 
void deleteNode(Node** head_ref, Node* del) 
{ 
	// base case 
	if (*head_ref == NULL || del == NULL) 
		return; 

	// If node to be deleted is head node 
	if (*head_ref == del) 
		*head_ref = del->next; 

	// Change next only if node to be 
	// deleted is NOT the last node 
	if (del->next != NULL) 
		del->next->prev = del->prev; 

	// Change prev only if node to be 
	// deleted is NOT the first node 
	if (del->prev != NULL) 
		del->prev->next = del->next; 

	// Finally, free the memory occupied by del 
	free(del); 

	return; 
} 

// function to delete all the odd nodes 
// from the doubly linked list 
void deleteOddNodes(Node** head_ref) 
{ 
	Node* ptr = *head_ref; 
	Node* next; 

	while (ptr != NULL) { 
		next = ptr->next; 
		// if true, delete node 'ptr' 
		if (ptr->data % 2 != 0) 
			deleteNode(head_ref, ptr); 
		ptr = next; 
	} 
} 

// function to print nodes in a 
// given doubly linked list 
void printList(Node* head) 
{ 
	while (head != NULL) { 
		cout << head->data << " "; 
		head = head->next; 
	} 
} 

// Driver program 
int main() 
{ 
	// start with the empty list 
	Node* head = NULL; 

	// create the doubly linked list 
	// 15 <-> 16 <-> 7 <-> 6 <-> 17 
	push(&head, 17); 
	push(&head, 6); 
	push(&head, 7); 
	push(&head, 16); 
	push(&head, 15); 

	cout << "Original List: "; 
	printList(head); 

	deleteOddNodes(&head); 

	cout << "\nModified List: "; 
	printList(head); 
}

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