# 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);
}
```

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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