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.

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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#

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