Delete all the even nodes from a Doubly Linked List

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

Examples:



Input: Initial List = 15 <=> 16 <=> 6 <=> 7 <=> 17
Output: Final List = 15 <=> 7 <=> 17

Input: Initial List = 5 <=> 3 <=> 6 <=> 8 <=> 4 <=> 1 <=> 2 <=> 9
Output: Final List = 5 <=> 3 <=> 1 <=> 9

The idea is to traverse the nodes of the doubly linked list one by one and get the pointer of the nodes having data even. Delete those nodes by following the approach used in this post.

Below is the implementation of above idea:

C++

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// C++ implementation to delete all
// the even 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 even nodes
// from the doubly linked list
void deleteEvenNodes(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);
  
    deleteEvenNodes(&head);
  
    cout << "\nModified List: ";
    printList(head);
}

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Java

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// Java implementation to delete all 
// the even nodes from the doubly 
// linked list 
class GFG
{
      
// Node of the doubly linked list 
static class Node 
    int data; 
    Node prev, next; 
}; 
  
// function to insert a node at the beginning 
// of the Doubly Linked List 
static Node push(Node head_ref, int new_data) 
    // allocate node 
    Node new_node = new 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;
    return head_ref;
  
// function to delete a node in a Doubly Linked List. 
// head_ref -. pointer to head node pointer. 
// del -. pointer to node to be deleted 
static Node deleteNode(Node head_ref, Node del) 
    // base case 
    if (head_ref == null || del == null
        return null
  
    // 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; 
  
  
    return head_ref; 
  
// function to delete all the even nodes 
// from the doubly linked list 
static Node deleteEvenNodes(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; 
    
    return head_ref;
  
// function to print nodes in a 
// given doubly linked list 
static void printList(Node head) 
    while (head != null)
    
        System.out.print( head.data + " "); 
        head = head.next; 
    
  
// Driver code 
public static void main(String args[])
    // start with the empty list 
    Node head = null
  
    // create the doubly linked list 
    // 15 <. 16 <. 7 <. 6 <. 17 
    head = push(head, 17); 
    head = push(head, 6); 
    head = push(head, 7); 
    head = push(head, 16); 
    head = push(head, 15); 
  
    System.out.print("Original List: "); 
    printList(head); 
  
    head=deleteEvenNodes(head); 
  
    System.out.print( "\nModified List: "); 
    printList(head); 
}
  
// This code is contributed by Arnab Kundu

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Output:

Original List: 15 16 7 6 17 
Modified List: 15 7 17

Time Complexity: O(N), where N is the total number of nodes.



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