Delete alternate nodes of a Linked List

Given a Singly Linked List, starting from the second node delete all alternate nodes of it. For example, if the given linked list is 1->2->3->4->5 then your function should convert it to 1->3->5, and if the given linked list is 1->2->3->4 then convert it to 1->3.

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

Method 1 (Iterative)
Keep track of previous of the node to be deleted. First change the next link of previous node and then free the memory allocated for the node.

C++

// C++ program to remove alternate  
// nodes of a linked list  
#include <bits/stdc++.h> 
using namespace std; 
  
/* A linked list node */
class Node  
{  
    public: 
    int data;  
    Node *next;  
};  
  
/* deletes alternate nodes  
of a list starting with head */
void deleteAlt(Node *head)  
{  
    if (head == NULL)  
        return;  
  
    /* Initialize prev and node to be deleted */
    Node *prev = head;  
    Node *node = head->next;  
  
    while (prev != NULL && node != NULL)  
    {  
        /* Change next link of previous node */
        prev->next = node->next;  
  
        /* Free memory */
        free(node);  
  
        /* Update prev and node */
        prev = prev->next;  
        if (prev != NULL)  
            node = prev->next;  
    }  
}  
  
/* UTILITY FUNCTIONS TO TEST fun1() and fun2() */
/* Given a reference (pointer to pointer) to the head  
of a list and an int, push a new node on the front  
of the list. */
void push(Node** head_ref, int new_data)  
{  
    /* allocate node */
    Node* new_node = new Node(); 
  
    /* put in the data */
    new_node->data = new_data;  
  
    /* link the old list off the new node */
    new_node->next = (*head_ref);  
  
    /* move the head to point to the new node */
    (*head_ref) = new_node;  
}  
  
/* Function to print nodes in a given linked list */
void printList(Node *node)  
{  
    while (node != NULL)  
    {  
        cout<< node->data<<" ";  
        node = node->next;  
    }  
}  
  
/* Driver code */
int main()  
{  
    /* Start with the empty list */
    Node* head = NULL;  
  
    /* Using push() to construct below list  
    1->2->3->4->5 */
    push(&head, 5);  
    push(&head, 4);  
    push(&head, 3);  
    push(&head, 2);  
    push(&head, 1);  
  
    cout<<"List before calling deleteAlt() \n";  
    printList(head);  
  
    deleteAlt(head);  
  
    cout<<"\nList after calling deleteAlt() \n";  
    printList(head);  
  
    return 0;  
}  
  
// This code is contributed by rathbhupendra 

C

// C program to remove alternate nodes of a linked list 
#include<stdio.h> 
#include<stdlib.h> 
  
/* A linked list node */
struct Node 
{ 
    int data; 
    struct Node *next; 
}; 
  
/* deletes alternate nodes of a list starting with head */
void deleteAlt(struct Node *head) 
{ 
    if (head == NULL) 
        return; 
  
    /* Initialize prev and node to be deleted */
    struct Node *prev = head; 
    struct Node *node = head->next; 
  
    while (prev != NULL && node != NULL) 
    { 
        /* Change next link of previous node */
        prev->next = node->next; 
  
        /* Free memory */
        free(node); 
  
        /* Update prev and node */
        prev = prev->next; 
        if (prev != NULL) 
            node = prev->next; 
    } 
} 
  
/* UTILITY FUNCTIONS TO TEST fun1() and fun2() */
/* Given a reference (pointer to pointer) to the head 
  of a list and an int, push a new node on the front 
  of the list. */
void push(struct Node** head_ref, int new_data) 
{ 
    /* allocate node */
    struct Node* new_node = 
        (struct Node*) malloc(sizeof(struct Node)); 
  
    /* put in the data  */
    new_node->data  = new_data; 
  
    /* link the old list off the new node */
    new_node->next = (*head_ref); 
  
    /* move the head to point to the new node */
    (*head_ref)    = new_node; 
} 
  
/* Function to print nodes in a given linked list */
void printList(struct Node *node) 
{ 
    while (node != NULL) 
    { 
        printf("%d ", node->data); 
        node = node->next; 
    } 
} 
  
/* Driver program to test above functions */
int main() 
{ 
    /* Start with the empty list */
    struct Node* head = NULL; 
  
    /* Using push() to construct below list 
      1->2->3->4->5  */
    push(&head, 5); 
    push(&head, 4); 
    push(&head, 3); 
    push(&head, 2); 
    push(&head, 1); 
  
    printf("\nList before calling deleteAlt() \n"); 
    printList(head); 
  
    deleteAlt(head); 
  
    printf("\nList after calling deleteAlt() \n"); 
    printList(head); 
  
    return 0; 
} 

Java

// Java program to delete alternate nodes of a linked list 
class LinkedList 
{ 
    Node head;  // head of list 
   
    /* Linked list Node*/
    class Node 
    { 
        int data; 
        Node next; 
        Node(int d) {data = d; next = null; } 
    } 
  
    void deleteAlt() 
    { 
       if (head == null)  
          return; 
  
       Node prev = head; 
       Node now = head.next; 
  
       while (prev != null && now != null)  
       {            
           /* Change next link of previus node */
           prev.next = now.next; 
  
           /* Free node */
           now = null; 
  
           /*Update prev and now */
           prev = prev.next; 
           if (prev != null)  
              now = prev.next; 
       } 
    }                  
  
                      
    /* Utility functions */
  
    /* Inserts a new Node at front of the list. */
    public void push(int new_data) 
    { 
        /* 1 & 2: Allocate the Node & 
                  Put in the data*/
        Node new_node = new Node(new_data); 
   
        /* 3. Make next of new Node as head */
        new_node.next = head; 
   
        /* 4. Move the head to point to new Node */
        head = new_node; 
    } 
  
    /* Function to print linked list */
    void printList() 
    { 
        Node temp = head; 
        while(temp != null) 
        { 
           System.out.print(temp.data+" "); 
           temp = temp.next; 
        }   
        System.out.println(); 
    } 
  
     /* Driver program to test above functions */
    public static void main(String args[]) 
    { 
        LinkedList llist = new LinkedList(); 
          
        /* Constructed Linked List is 1->2->3->4->5->null */
        llist.push(5); 
        llist.push(4); 
        llist.push(3); 
        llist.push(2); 
        llist.push(1); 
          
        System.out.println("Linked List before calling deleteAlt() "); 
        llist.printList(); 
          
        llist.deleteAlt(); 
          
        System.out.println("Linked List after calling deleteAlt() "); 
        llist.printList(); 
    } 
}  
/* This code is contributed by Rajat Mishra */

Python3

# Python3 program to remove alternate  
# nodes of a linked list  
import math  
  
# A linked list node  
class Node:  
    def __init__(self, data):  
        self.data = data  
        self.next = None
          
# deletes alternate nodes  
# of a list starting with head  
def deleteAlt(head):  
    if (head == None): 
        return
  
    # Initialize prev and node to be deleted  
    prev = head  
    now = head.next
  
    while (prev != None and now != None):  
          
        # Change next link of previous node  
        prev.next = now.next
  
        # Free memory  
        now = None
  
        # Update prev and node  
        prev = prev.next
        if (prev != None):  
            now = prev.next
      
# UTILITY FUNCTIONS TO TEST fun1() and fun2()  
# Given a reference (poer to poer) to the head  
# of a list and an , push a new node on the front  
# of the list.  
def push(head_ref, new_data):  
      
    # allocate node  
    new_node = Node(new_data) 
  
    # put in the data  
    new_node.data = new_data  
  
    # link the old list off the new node  
    new_node.next = head_ref  
  
    # move the head to po to the new node  
    head_ref = new_node  
    return head_ref 
  
# Function to print nodes in a given linked list  
def prList(node):  
    while (node != None):  
        print(node.data, end = " ")  
        node = node.next
      
# Driver code  
if __name__=='__main__':  
      
    # Start with the empty list  
    head = None
  
    # Using head=push() to construct below list  
    # 1.2.3.4.5  
    head = push(head, 5)  
    head = push(head, 4)  
    head = push(head, 3)  
    head = push(head, 2)  
    head = push(head, 1)  
  
    print("List before calling deleteAlt() ") 
    prList(head)  
  
    deleteAlt(head)  
  
    print("\nList after calling deleteAlt() ")  
    prList(head)  
  
# This code is contributed by Srathore 

C#

// C# program to delete alternate 
// nodes of a linked list 
using System; 
  
public class LinkedList 
{ 
    Node head; // head of list 
  
    /* Linked list Node*/
    public class Node 
    { 
        public int data; 
        public Node next; 
        public Node(int d)  
        { 
            data = d; next = null;  
              
        } 
    } 
  
    void deleteAlt() 
    { 
        if (head == null)  
            return; 
  
        Node prev = head; 
        Node now = head.next; 
  
        while (prev != null && now != null)  
        {          
            /* Change next link of previus node */
            prev.next = now.next; 
  
            /* Free node */
            now = null; 
  
            /*Update prev and now */
            prev = prev.next; 
            if (prev != null)  
                now = prev.next; 
        } 
    }                  
  
                      
    /* Utility functions */
  
    /* Inserts a new Node at front of the list. */
    public void push(int new_data) 
    { 
        /* 1 & 2: Allocate the Node & 
                Put in the data*/
        Node new_node = new Node(new_data); 
  
        /* 3. Make next of new Node as head */
        new_node.next = head; 
  
        /* 4. Move the head to point to new Node */
        head = new_node; 
    } 
  
    /* Function to print linked list */
    void printList() 
    { 
        Node temp = head; 
        while(temp != null) 
        { 
        Console.Write(temp.data+" "); 
        temp = temp.next; 
        }  
        Console.WriteLine(); 
    } 
  
    /* Driver code*/
    public static void Main(String []args) 
    { 
        LinkedList llist = new LinkedList(); 
          
        /* Constructed Linked List is 
        1->2->3->4->5->null */
        llist.push(5); 
        llist.push(4); 
        llist.push(3); 
        llist.push(2); 
        llist.push(1); 
          
        Console.WriteLine("Linked List before" +  
                            "calling deleteAlt() "); 
        llist.printList(); 
          
        llist.deleteAlt(); 
          
        Console.WriteLine("Linked List after" +  
                            "calling deleteAlt() "); 
        llist.printList(); 
    } 
} 
  
// This code has been contributed  
// by 29AjayKumar 

Output:

List before calling deleteAlt() 
1 2 3 4 5 
List after calling deleteAlt() 
1 3 5

Time Complexity: O(n) where n is the number of nodes in the given Linked List.

Method 2 (Recursive)
Recursive code uses the same approach as method 1. The recursive coed is simple and short, but causes O(n) recursive function calls for a linked list of size n.

C++

/* deletes alternate nodes of a list starting with head */
void deleteAlt(Node *head)  
{  
    if (head == NULL)  
        return;  
  
    Node *node = head->next;  
  
    if (node == NULL)  
        return;  
  
    /* Change the next link of head */
    head->next = node->next;  
  
    /* free memory allocated for node */
    free(node);  
  
    /* Recursively call for the new next of head */
    deleteAlt(head->next);  
}  
  
// This code is contributed by rathbhupendra 

C

/* deletes alternate nodes of a list starting with head */
void deleteAlt(struct Node *head) 
{ 
    if (head == NULL) 
        return; 
  
    struct Node *node = head->next; 
  
    if (node == NULL) 
        return; 
  
    /* Change the next link of head */
    head->next = node->next; 
  
    /* free memory allocated for node */
    free(node); 
  
    /* Recursively call for the new next of head */
    deleteAlt(head->next); 
} 

Java

/* deletes alternate nodes of a list 
starting with head */
static Node deleteAlt(Node head)  
{  
    if (head == null)  
        return;  
  
    Node node = head.next;  
  
    if (node == null)  
        return;  
  
    /* Change the next link of head */
    head.next = node.next;  
  
  
    /* Recursively call for the new next of head */
    head.next = deleteAlt(head.next);  
}  
  
// This code is contributed by Arnab Kundu 

Python3

# deletes alternate nodes of a list starting with head  
def deleteAlt(head):  
    if (head == None):  
        return
  
    node = head.next
  
    if (node == None):  
        return
  
    # Change the next link of head  
    head.next = node.next
  
    # free memory allocated for node  
    #free(node)  
  
    # Recursively call for the new next of head  
    deleteAlt(head.next)  
  
# This code is contributed by Srathore 

C#

/* deletes alternate nodes of a list 
starting with head */
static Node deleteAlt(Node head)  
{  
    if (head == null)  
        return;  
  
    Node node = head.next;  
  
    if (node == null)  
        return;  
  
    /* Change the next link of head */
    head.next = node.next;  
  
  
    /* Recursively call for the new next of head */
    head.next = deleteAlt(head.next);  
}  
  
// This code is contributed by Arnab Kundu 

Time Complexity: O(n)

Please write comments if you find the above code/algorithm incorrect, or find better ways to solve the same problem.

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.

My Personal Notes

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

C++

C

Java

Python3

C#

C++

C

Java

Python3

C#

Recommended Posts: