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


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:

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// C code to print the sum of Alternate Nodes
#include <stdio.h>
#include <stdlib.h>
  
/* 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;
}

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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:

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// CPP code to print sum of alternate nodes
// of a linked list using recursion
#include <bits/stdc++.h>
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;
}

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Output:

31


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