# Sum of the alternate nodes of linked list

Given a linked list, the task is to print the sum of the alternate nodes of the linked list.

Examples:

```Input : 1 -> 8 -> 3 -> 10 -> 17 -> 22 -> 29 -> 42
Output : 50
Alternate nodes : 1 -> 3 -> 17 -> 29

Input : 10 -> 17 -> 33 -> 38 -> 73
Output : 116
Alternate nodes : 10 -> 33 -> 73
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Iterative Approach :

1. Traverse the whole linked list.
2. Set sum = 0 and count=0.
3. Add the data of node to sum when count is even.
4. Visit the next node.

Below is the implementation of this approach:

## C++

 `// C++ code to print the sum of Alternate Nodes  ` `#include ` `using` `namespace` `std; ` ` `  `/* Link list node */` `struct` `Node ` `{  ` `    ``int` `data;  ` `    ``struct` `Node* next;  ` `};  ` ` `  `/* Function to get the alternate  ` `nodes of the linked list */` `int` `sumAlternateNode(``struct` `Node* head)  ` `{  ` `    ``int` `count = 0;  ` `    ``int` `sum = 0;  ` ` `  `    ``while` `(head != NULL)  ` `    ``{  ` ` `  `        ``// when count is even sum the nodes  ` `        ``if` `(count % 2 == 0)  ` `            ``sum += head->data;  ` ` `  `        ``// count the nodes  ` `        ``count++;  ` ` `  `        ``// move on the next node.  ` `        ``head = head->next;  ` `    ``}  ` `    ``return` `sum;  ` `}  ` ` `  `// Function to push node at head  ` `void` `push(``struct` `Node** head_ref, ``int` `new_data)  ` `{  ` `    ``struct` `Node* new_node =  ` `            ``(``struct` `Node*)``malloc``(``sizeof``(``struct` `Node));  ` `    ``new_node->data = new_data;  ` `    ``new_node->next = (*head_ref);  ` `    ``(*head_ref) = new_node;  ` `}  ` ` `  `// Driver code  ` `int` `main()  ` `{  ` `    ``/* Start with the empty list */` `    ``struct` `Node* head = NULL;  ` ` `  `    ``/* Use push() function to construct  ` `    ``the below list 8 -> 23 -> 11 -> 29 -> 12 */` `    ``push(&head, 12);  ` `    ``push(&head, 29);  ` `    ``push(&head, 11);  ` `    ``push(&head, 23);  ` `    ``push(&head, 8);  ` ` `  `    ``cout << sumAlternateNode(head);  ` `    ``return` `0;  ` `}  ` ` `  `// This code is contributed by SHUBHAMSINGH10 `

## C

 `// C code to print the sum of Alternate Nodes ` `#include ` `#include ` ` `  `/* Link list node */` `struct` `Node { ` `    ``int` `data; ` `    ``struct` `Node* next; ` `}; ` ` `  `/* Function to get the alternate  ` `   ``nodes of the linked list */` `int` `sumAlternateNode(``struct` `Node* head) ` `{ ` `    ``int` `count = 0; ` `    ``int` `sum = 0; ` ` `  `    ``while` `(head != NULL) { ` ` `  `        ``// when count is even sum the nodes ` `        ``if` `(count % 2 == 0) ` `            ``sum += head->data; ` ` `  `        ``// count the nodes ` `        ``count++; ` ` `  `        ``// move on the next node. ` `        ``head = head->next; ` `    ``} ` `    ``return` `sum; ` `} ` ` `  `// Function to push node at head ` `void` `push(``struct` `Node** head_ref, ``int` `new_data) ` `{ ` `    ``struct` `Node* new_node = (``struct` `Node*)``malloc``(``sizeof``(``struct` `Node)); ` `    ``new_node->data = new_data; ` `    ``new_node->next = (*head_ref); ` `    ``(*head_ref) = new_node; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``/* Start with the empty list */` `    ``struct` `Node* head = NULL; ` ` `  `    ``/* Use push() function to construct  ` `       ``the below list 8 -> 23 -> 11 -> 29 -> 12 */` `    ``push(&head, 12); ` `    ``push(&head, 29); ` `    ``push(&head, 11); ` `    ``push(&head, 23); ` `    ``push(&head, 8); ` ` `  `    ``printf``(``" %d "``, sumAlternateNode(head)); ` `    ``return` `0; ` `} `

## Java

 `// Java code to print the sum of Alternate Nodes  ` `class` `GFG ` `{ ` `     `  `/* Link list node */` `static` `class` `Node  ` `{  ` `    ``int` `data;  ` `    ``Node next;  ` `};  ` ` `  `/* Function to get the alternate  ` `nodes of the linked list */` `static` `int` `sumAlternateNode( Node head)  ` `{  ` `    ``int` `count = ``0``;  ` `    ``int` `sum = ``0``;  ` ` `  `    ``while` `(head != ``null``)  ` `    ``{  ` ` `  `        ``// when count is even sum the nodes  ` `        ``if` `(count % ``2` `== ``0``)  ` `            ``sum += head.data;  ` ` `  `        ``// count the nodes  ` `        ``count++;  ` ` `  `        ``// move on the next node.  ` `        ``head = head.next;  ` `    ``}  ` `    ``return` `sum;  ` `}  ` ` `  `// Function to push node at head  ` `static` `Node push( Node head_ref, ``int` `new_data)  ` `{  ` `    ``Node new_node =``new` `Node(); ` `    ``new_node.data = new_data;  ` `    ``new_node.next = (head_ref);  ` `    ``(head_ref) = new_node;  ` `    ``return` `head_ref; ` `}  ` ` `  `// Driver code  ` `public` `static` `void` `main(String args[]) ` `{  ` `    ``/* Start with the empty list */` `    ``Node head = ``null``;  ` ` `  `    ``/* Use push() function to con  ` `    ``the below list 8 . 23 . 11 . 29 . 12 */` `    ``head = push(head, ``12``);  ` `    ``head = push(head, ``29``);  ` `    ``head = push(head, ``11``);  ` `    ``head = push(head, ``23``);  ` `    ``head = push(head, ``8``);  ` ` `  `    ``System.out.printf(``" %d "``, sumAlternateNode(head));  ` `}  ` `} ` ` `  `// This code is contributed by Arnab Kundu `

## Python3

 `# Python3 code to print the ` `# sum of Alternate Nodes ` `import` `math ` ` `  `# Link list node  ` `class` `Node:  ` `    ``def` `__init__(``self``, data):  ` `        ``self``.data ``=` `data  ` `        ``self``.``next` `=` `None` ` `  `# Function to get the alternate  ` `# nodes of the linked list  ` `def` `sumAlternateNode(head): ` `    ``count ``=` `0` `    ``sum` `=` `0` ` `  `    ``while` `(head !``=` `None``): ` ` `  `        ``# when count is even sum the nodes ` `        ``if` `(count ``%` `2` `=``=` `0``): ` `            ``sum` `=` `sum` `+` `head.data ` ` `  `        ``# count the nodes ` `        ``count ``=` `count ``+` `1` ` `  `        ``# move on the next node. ` `        ``head ``=` `head.``next` `     `  `    ``return` `sum` ` `  `# Function to push node at head ` `def` `push(head_ref, new_data): ` `    ``new_node ``=` `Node(new_data) ` `    ``new_node.data ``=` `new_data ` `    ``new_node.``next` `=` `head_ref ` `    ``head_ref ``=` `new_node ` `    ``return` `head_ref ` ` `  `# Driver code ` `if` `__name__``=``=``'__main__'``:  ` ` `  `    ``# Start with the empty list  ` `    ``head ``=` `None` ` `  `    ``# Use push() function to construct  ` `    ``# the below list 8 . 23 . 11 . 29 . 12  ` `    ``head ``=` `push(head, ``12``)  ` `    ``head ``=` `push(head, ``29``)  ` `    ``head ``=` `push(head, ``11``)  ` `    ``head ``=` `push(head, ``23``)  ` `    ``head ``=` `push(head, ``8``)  ` ` `  `    ``print``(sumAlternateNode(head)) ` `     `  `# This code is contributed by Srathore `

## C#

 `// C# code to print the sum of Alternate Nodes  ` `using` `System; ` ` `  `class` `GFG  ` `{  ` `     `  `/* Link list node */` `public` `class` `Node  ` `{  ` `    ``public` `int` `data;  ` `    ``public` `Node next;  ` `};  ` ` `  `/* Function to get the alternate  ` `nodes of the linked list */` `static` `int` `sumAlternateNode( Node head)  ` `{  ` `    ``int` `count = 0;  ` `    ``int` `sum = 0;  ` ` `  `    ``while` `(head != ``null``)  ` `    ``{  ` ` `  `        ``// when count is even sum the nodes  ` `        ``if` `(count % 2 == 0)  ` `            ``sum += head.data;  ` ` `  `        ``// count the nodes  ` `        ``count++;  ` ` `  `        ``// move on the next node.  ` `        ``head = head.next;  ` `    ``}  ` `    ``return` `sum;  ` `}  ` ` `  `// Function to push node at head  ` `static` `Node push( Node head_ref, ``int` `new_data)  ` `{  ` `    ``Node new_node =``new` `Node();  ` `    ``new_node.data = new_data;  ` `    ``new_node.next = (head_ref);  ` `    ``(head_ref) = new_node;  ` `    ``return` `head_ref;  ` `}  ` ` `  `// Driver code  ` `public` `static` `void` `Main(String []args)  ` `{  ` `    ``/* Start with the empty list */` `    ``Node head = ``null``;  ` ` `  `    ``/* Use push() function to con  ` `    ``the below list 8 . 23 . 11 . 29 . 12 */` `    ``head = push(head, 12);  ` `    ``head = push(head, 29);  ` `    ``head = push(head, 11);  ` `    ``head = push(head, 23);  ` `    ``head = push(head, 8);  ` ` `  `    ``Console.Write(``" {0} "``, sumAlternateNode(head));  ` `}  ` `}  ` ` `  `// This code has been contributed by 29AjayKumar `

Output:

```31
```

Recursive Approach :

1. Initialize a static variable(say flag).
2. If flag is odd, add the node with sum.
3. increase head and flag by 1, and recurse for next nodes.

Below is the implementation of this approach:

## C++

 `// CPP code to print sum of alternate nodes ` `// of a linked list using recursion ` `#include ` `using` `namespace` `std; ` ` `  `// A linked list node ` `struct` `Node { ` `    ``int` `data; ` `    ``struct` `Node* next; ` `}; ` ` `  `// Inserting node at the beginning ` `void` `push(``struct` `Node** head_ref, ``int` `new_data) ` `{ ` `    ``struct` `Node* new_node = (``struct` `Node*)``malloc``(``sizeof``(``struct` `Node)); ` `    ``new_node->data = new_data; ` `    ``new_node->next = (*head_ref); ` `    ``(*head_ref) = new_node; ` `} ` ` `  `// Function to find sum of alternate ` `// nodes of linked list. ` `// The boolean flag isOdd is used to find ` `// if the current node is even or odd. ` `void` `productAlternateNodes(``struct` `Node* node, ` `                           ``int``& sum, ``bool` `isOdd = ``true``) ` `{ ` `    ``if` `(node == NULL) ` `        ``return``; ` ` `  `    ``if` `(isOdd == ``true``) ` `        ``sum = sum + (node->data); ` ` `  `    ``productAlternateNodes(node->next, sum, !isOdd); ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``// Start with the empty list ` `    ``struct` `Node* head = NULL; ` ` `  `    ``// construct below list ` `    ``// 8 -> 23 -> 11 -> 29 -> 12 ` ` `  `    ``push(&head, 12); ` `    ``push(&head, 29); ` `    ``push(&head, 11); ` `    ``push(&head, 23); ` `    ``push(&head, 8); ` ` `  `    ``int` `sum = 0; ` ` `  `    ``productAlternateNodes(head, sum); ` ` `  `    ``cout << sum; ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java code to print sum of alternate nodes ` `// of a linked list using recursion ` ` `  `class` `GFG  ` `{ ` `     `  `// Start with the empty list  ` `static` `Node head=``null``;  ` ` `  `static` `int` `sum; ` ` `  `// A linked list node ` `static` `class` `Node  ` `{ ` `    ``int` `data; ` `    ``Node next; ` `}; ` ` `  `// Inserting node at the beginning ` `static` `Node push(Node head_ref, ``int` `new_data) ` `{ ` `    ``Node new_node = ``new` `Node(); ` `    ``new_node.data = new_data; ` `    ``new_node.next = head_ref; ` `    ``head_ref = new_node; ` `    ``return` `head=head_ref; ` `} ` ` `  `// Function to find sum of alternate ` `// nodes of linked list. ` `// The boolean flag isOdd is used to find ` `// if the current node is even or odd. ` `static` `void` `productAlternateNodes(Node node, ``boolean` `isOdd) ` `{ ` `    ``if` `(node == ``null``) ` `        ``return``; ` ` `  `    ``if` `(isOdd == ``true``) ` `        ``sum = sum + (node.data); ` ` `  `    ``productAlternateNodes(node.next, !isOdd); ` `} ` ` `  `// Driver code ` `public` `static` `void` `main(String[] args)  ` `{ ` `    ``// conbelow list ` `    ``// 8 . 23 . 11 . 29 . 12 ` ` `  `    ``push(head, ``12``); ` `    ``push(head, ``29``); ` `    ``push(head, ``11``); ` `    ``push(head, ``23``); ` `    ``push(head, ``8``); ` ` `  `    ``sum = ``0``; ` ` `  `    ``productAlternateNodes(head,``true``); ` ` `  `    ``System.out.println(sum); ` `} ` `} ` ` `  `/* This code is contributed by PrinciRaj1992 */`

## Python

 `# Python code to print sum of alternate nodes ` `# of a linked list using recursion ` ` `  `# Link list node  ` `class` `Node:  ` `     `  `    ``def` `__init__(``self``, data):  ` `        ``self``.data ``=` `data  ` `        ``self``.``next` `=` `next` `         `  `# function to insert a node at the ` `# beginning of the linked list ` `def` `push(head_ref, new_data): ` ` `  `    ``# allocate node  ` `    ``new_node ``=` `Node(``0``) ` ` `  `    ``# put in the data  ` `    ``new_node.data ``=` `new_data ` ` `  `    ``# link the old list to the new node  ` `    ``new_node.``next` `=` `(head_ref) ` ` `  `    ``# move the head to point to the new node  ` `    ``(head_ref) ``=` `new_node ` `     `  `    ``return` `head_ref ` `     `  `sum` `=` `0``; ` ` `  `# Function to find sum of alternate ` `# nodes of linked list. ` `# The boolean flag isOdd is used to find ` `# if the current node is even or odd. ` `def` `subtractAlternateNodes(node, isOdd ): ` ` `  `    ``global` `sum` `     `  `    ``if` `(node ``=``=` `None``): ` `        ``return``; ` ` `  `    ``if` `(isOdd ``=``=` `True``):  ` `        ``if` `(``sum` `=``=` `0``) : ` `            ``sum` `=` `node.data; ` `         `  `        ``else` `: ` `            ``sum` `=` `sum` `+` `(node.data); ` `         `  `    ``subtractAlternateNodes(node.``next``, ``not` `isOdd); ` ` `  `# Driver code ` ` `  `# Start with the empty list ` `head ``=` `None``; ` ` `  `# Construct below list ` `# 8 -> 23 -> 11 -> 29 -> 12 ` ` `  `head ``=` `push(head, ``12``); ` `head ``=` `push(head, ``29``); ` `head ``=` `push(head, ``11``); ` `head ``=` `push(head, ``23``); ` `head ``=` `push(head, ``8``); ` ` `  `sum` `=` `0``; ` ` `  `subtractAlternateNodes(head, ``True``); ` ` `  `print``( ``sum``); ` ` `  `# This code is contributed by Arnab Kundu `

## C#

 `// C# code to print sum of alternate nodes ` `// of a linked list using recursion ` `using` `System;  ` `     `  `class` `GFG  ` `{ ` `     `  `// Start with the empty list  ` `static` `Node head = ``null``;  ` ` `  `static` `int` `sum; ` ` `  `// A linked list node ` `public` `class` `Node  ` `{ ` `    ``public` `int` `data; ` `    ``public` `Node next; ` `}; ` ` `  `// Inserting node at the beginning ` `static` `Node push(Node head_ref, ``int` `new_data) ` `{ ` `    ``Node new_node = ``new` `Node(); ` `    ``new_node.data = new_data; ` `    ``new_node.next = head_ref; ` `    ``head_ref = new_node; ` `    ``return` `head=head_ref; ` `} ` ` `  `// Function to find sum of alternate ` `// nodes of linked list. ` `// The boolean flag isOdd is used to find ` `// if the current node is even or odd. ` `static` `void` `productAlternateNodes(Node node, ``bool` `isOdd) ` `{ ` `    ``if` `(node == ``null``) ` `        ``return``; ` ` `  `    ``if` `(isOdd == ``true``) ` `        ``sum = sum + (node.data); ` ` `  `    ``productAlternateNodes(node.next, !isOdd); ` `} ` ` `  `// Driver code ` `public` `static` `void` `Main(String[] args)  ` `{ ` `    ``// conbelow list ` `    ``// 8 -> 23 -> 11 -> 29 -> 12 ` ` `  `    ``push(head, 12); ` `    ``push(head, 29); ` `    ``push(head, 11); ` `    ``push(head, 23); ` `    ``push(head, 8); ` ` `  `    ``sum = 0; ` ` `  `    ``productAlternateNodes(head,``true``); ` ` `  `    ``Console.WriteLine(sum); ` `} ` `} ` ` `  `// This code is contributed by Rajput-Ji `

Output:

```31
```

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