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.

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 Programming:

// 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;
}

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 Programming:

/* 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);
}

Time Complexity: O(n)

Categorized in:

Linked ListSingly Linked List

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