# C Program To 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.

Method 1 (Iterative):
Keep track of previous of the node to be deleted. First, change the next link of the previous node and iteratively move to the next node.

## C

 `// C program to remove alternate nodes ` `// of a linked list` `#include` `#include`   `// 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 of 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 code` `int` `main()` `{` `    ``// Start with the empty list ` `    ``struct` `Node* head = NULL;`   `    ``/* Using push() to construct 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.

Auxiliary Space: O(1) because it is using constant space

Method 2 (Recursive):
Recursive code uses the same approach as method 1. The recursive code 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(``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)

Auxiliary space: O(n) for call stack because using recursion

Please refer complete article on Delete alternate nodes of a Linked List for more details!

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