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++

// 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; 
    } 
} 
  
/* Drier 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(); 
    } 
  
     /* Drier 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 */

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.

/* 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); 
} 