Data Modelling & AI Data Structure & Algorithm

Sudo Placement[1.4] | K Sum

By Dominic

23 June 2025

0

Given head of a linked list of integers and an integer k, your task is to modify the linked list as follows:

Consider nodes in groups of size k. In every group, replace value of first node with group sum.
Also, delete the elements of group except the first node.
During traversal, if the remaining nodes in linked list are less than k then also do the above considering remaining nodes.

Examples:

Input: N = 6, K = 2
1->2->3->4->5->6
Output: 3 7 11
We have denoted grouping of k elements by (). The elements inside () are summed.
1 -> 2 -> 3 -> 4 -> 5 -> 6 -> null
(1 -> 2) -> 3 -> 4 -> 5 -> 6 -> null
(3) -> 3 -> 4 -> 5 -> 6 -> null
3 -> (3 -> 4) -> 5 -> 6 -> null
3 -> (7) -> 5 -> 6 -> null
3 -> 7 -> (5 -> 6) -> null
3 -> 7 -> (11) -> null
3 -> 7 -> 11 -> null

Approach:

Insert the given nodes in the Linked list. The approach of insertion has been discussed in this post. Once the nodes are inserted, iterate from the beginning of the list. Mark the first node as temp node. Iterate for the next k-1 nodes and sum up the nodes in sum variable. If the number of nodes is less than K, then replace the temp node’s data with sum and point temp to NULL.

If there is a group with K nodes, replace the temp’s data with sum and move temp to the node which is just after K nodes. Continue the above operation till temp points to NULL. Once temp reaches the end, it means all groups of size K has been traversed and the node has been replaced with the sum of nodes of size K. Once the operation of replacements have been done, the linked list thus formed can be printed. The method of printing the linked list has been discussed in this post.

Below is the implementation of the above approach:

C++

// C++ program for
// SP - K Sum
 
#include <iostream>
using namespace std;
 
// structure for linked list
struct Node {
    long long data;
    Node* next;
};
 
// allocated new node
Node* newNode(int key)
{
    Node* temp = new Node;
    temp->data = key;
    temp->next = NULL;
    return temp;
}
 
// function to insert node in a Linked List
Node* insertNode(Node* head, int key)
{
    if (head == NULL)
        head = newNode(key);
    else
        head->next = insertNode(head->next, key);
 
    return head;
}
 
// traverse the node
// to print it
void print(Node* head)
{
    // traverse the linked list
    while (head) {
        cout << head->data << " ";
        head = head->next;
    }
    cout << endl;
}
 
// function to perform the following operation
void KSum(Node* head, int k)
{
    // initially pointing to start
    Node* temp = head;
 
    // iterate till end
    while (temp) {
 
        // dummy variable to store k
        long long count = k;
 
        // summation of nodes
        long long sum = 0;
 
        // currently at node from
        // which iteration is done
        Node* curr = temp;
 
        // till k nodes are visited and
        // last node is not visited
        while (count-- && curr) {
            // add to sum the node data
            sum = sum + curr->data;
 
            // move to the next node
            curr = curr->next;
        }
 
        // change pointer's data of temp to sum
        temp->data = sum;
 
        // move temp to next pointer
        temp->next = curr;
 
        // move temp to the next pointer
        temp = temp->next;
    }
}
 
// Driver Code
int main()
{
 
    Node* head = NULL;
 
    // inserts nodes in the linked list
    head = insertNode(head, 1);
    head = insertNode(head, 2);
    head = insertNode(head, 3);
    head = insertNode(head, 4);
    head = insertNode(head, 5);
    head = insertNode(head, 6);
 
    int k = 2;
    // Function call to perform the
    // given operations
    KSum(head, k);
 
    // print the linked list
    print(head);
 
    return 0;
}

Java

// Java program for
// SP - K Sum
 
import java.io.*;
import java.util.*;
 
// structure for linked list
class Node 
{
    int data;
    Node next;
    Node(int key)
    {
        data = key;
        next = null;
    }
}
 
class GFG
{
     
// allocated new node
static Node head;
 
// function to insert node
// in a Linked List
public static Node insertNode(Node head, int key)
{
    if (head == null)
        head = new Node(key);
    else
        head.next = insertNode(head.next, key);
 
    return head;
}
 
// traverse the node
// to print it
public static void print(Node head)
{
    // traverse the linked list
    while (head != null) 
    {
        System.out.print(head.data + " ");
        head = head.next;
    }
    System.out.println();
}
 
// function to perform 
// the following operation
public static void KSum(Node head, int k)
{
    // initially pointing
    // to start
    Node temp = head;
 
    // iterate till end
    while (temp != null) 
    {
 
        // dummy variable
        // to store k
        int count = k;
 
        // summation of nodes
        int sum = 0;
 
        // currently at node from
        // which iteration is done
        Node curr = temp;
 
        // till k nodes are visited and
        // last node is not visited
        while (count > 0 && curr != null) 
        {
            // add to sum the node data
            sum = sum + curr.data;
 
            // move to the next node
            curr = curr.next;
            count--;
        }
 
        // change pointer's data
        // of temp to sum
        temp.data = sum;
 
        // move temp to
        // next pointer
        temp.next = curr;
 
        // move temp to 
        // the next pointer
        temp = temp.next;
    }
     
    //return temp;
}
 
// Driver Code
public static void main(String args[])
{
     head = null;
 
    // inserts nodes in
    // the linked list
    head = insertNode(head, 1);
    head = insertNode(head, 2);
    head = insertNode(head, 3);
    head = insertNode(head, 4);
    head = insertNode(head, 5);
    head = insertNode(head, 6);
 
    int k = 2;
     
    // Function call to perform 
    // the given operations
     KSum(head, k);
 
    // print the linked list
    print(head);
}
}

Python3

# Python program for
# SP - K Sum
 
# structure for linked list
class Node:
    def __init__(self, key):
        self.data = key
        self.next = None
 
# function to insert node in a Linked List
def insertNode(head: Node, key: int) -> Node:
    if head is None:
        head = Node(key)
    else:
        head.next = insertNode(head.next, key)
 
    return head
 
# traverse the node
# to print it
def Print(head: Node):
 
    # traverse the linked list
    while head:
        print(head.data, end = " ")
        head = head.next
    print()
 
# function to perform the following operation
def KSum(head: Node, k: int):
 
    # initially pointing to start
    temp = head
 
    # iterate till end
    while temp:
 
        # dummy variable to store k
        count = k
 
        # summation of nodes
        sum = 0
 
        # currently at node from
        # which iteration is done
        curr = temp
 
        # till k nodes are visited and
        # last node is not visited
        while count and curr:
 
            # add to sum the node data
            sum = sum + curr.data
 
            # move to the next node
            curr = curr.next
            count -= 1
 
        # change pointer's data of temp to sum
        temp.data = sum
 
        # move temp to next pointer
        temp.next = curr
 
        # move temp to the next pointer
        temp = temp.next
 
# Driver Code
if __name__ == "__main__":
    head = None
 
    # inserts nodes in the linked list
    head = insertNode(head, 1)
    head = insertNode(head, 2)
    head = insertNode(head, 3)
    head = insertNode(head, 4)
    head = insertNode(head, 5)
    head = insertNode(head, 6)
 
    k = 2
 
    # Function call to perform the
    # given operations
    KSum(head, k)
 
    # print the linked list
    Print(head)
 
# This code is contributed by
# sanjeev2552

C#

// C# program for SP - K Sum
using System;
 
// structure for linked list
public class Node 
{
    public int data;
    public Node next;
    public Node(int key)
    {
        data = key;
        next = null;
    }
}
 
public class GFG
{
     
// allocated new node
static Node head;
 
// function to insert node
// in a Linked List
public static Node insertNode(Node head, int key)
{
    if (head == null)
        head = new Node(key);
    else
        head.next = insertNode(head.next, key);
 
    return head;
}
 
// traverse the node
// to print it
public static void print(Node head)
{
    // traverse the linked list
    while (head != null) 
    {
        Console.Write(head.data + " ");
        head = head.next;
    }
    Console.WriteLine();
}
 
// function to perform 
// the following operation
public static void KSum(Node head, int k)
{
    // initially pointing
    // to start
    Node temp = head;
 
    // iterate till end
    while (temp != null) 
    {
 
        // dummy variable
        // to store k
        int count = k;
 
        // summation of nodes
        int sum = 0;
 
        // currently at node from
        // which iteration is done
        Node curr = temp;
 
        // till k nodes are visited and
        // last node is not visited
        while (count > 0 && curr != null) 
        {
            // add to sum the node data
            sum = sum + curr.data;
 
            // move to the next node
            curr = curr.next;
            count--;
        }
 
        // change pointer's data
        // of temp to sum
        temp.data = sum;
 
        // move temp to
        // next pointer
        temp.next = curr;
 
        // move temp to 
        // the next pointer
        temp = temp.next;
    }
    // return temp;
}
 
// Driver Code
public static void Main()
{
    head = null;
 
    // inserts nodes in
    // the linked list
    head = insertNode(head, 1);
    head = insertNode(head, 2);
    head = insertNode(head, 3);
    head = insertNode(head, 4);
    head = insertNode(head, 5);
    head = insertNode(head, 6);
 
    int k = 2;
     
    // Function call to perform 
    // the given operations
    KSum(head, k);
 
    // print the linked list
    print(head);
}
}
 
/* This code contributed by PrinciRaj1992 */

Javascript

<script>
      // JavaScript program for SP - K Sum
      // structure for linked list
      class Node
      {
        constructor(key)
        {
          this.data = key;
          this.next = null;
        }
      }
 
      // allocated new node
      var head = null;
 
      // function to insert node
      // in a Linked List
      function insertNode(head, key)
      {
        if (head == null) head = new Node(key);
        else head.next = insertNode(head.next, key);
 
        return head;
      }
 
      // traverse the node
      // to print it
      function print(head) 
      {
       
        // traverse the linked list
        while (head != null)
        {
          document.write(head.data + " ");
          head = head.next;
        }
        document.write("<br>");
      }
 
      // function to perform
      // the following operation
      function KSum(head, k)
      {
       
        // initially pointing
        // to start
        var temp = head;
 
        // iterate till end
        while (temp != null) 
        {
         
          // dummy variable
          // to store k
          var count = k;
 
          // summation of nodes
          var sum = 0;
 
          // currently at node from
          // which iteration is done
          var curr = temp;
 
          // till k nodes are visited and
          // last node is not visited
          while (count > 0 && curr != null)
          {
           
            // add to sum the node data
            sum = sum + curr.data;
 
            // move to the next node
            curr = curr.next;
            count--;
          }
 
          // change pointer's data
          // of temp to sum
          temp.data = sum;
 
          // move temp to
          // next pointer
          temp.next = curr;
 
          // move temp to
          // the next pointer
          temp = temp.next;
        }
         
        // return temp;
      }
 
      // Driver Code
      head = null;
 
      // inserts nodes in
      // the linked list
      head = insertNode(head, 1);
      head = insertNode(head, 2);
      head = insertNode(head, 3);
      head = insertNode(head, 4);
      head = insertNode(head, 5);
      head = insertNode(head, 6);
 
      var k = 2;
 
      // Function call to perform
      // the given operations
      KSum(head, k);
 
      // print the linked list
      print(head);
       
      // This code is contributed by rdtank.
    </script>

Output

3 7 11

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Sudo Placement[1.4] | K Sum

C++

Java

Python3

C#

Javascript

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