Maximum sum of K consecutive nodes in the given Linked List

Given a linked list, the task is tofind the maximum sum obtained by adding any k consecutive nodes of linked list.

Examples:

Input: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> NULL, K = 5
Output: 20
Maximum sum is obtained by adding last 5 nodes

Input: 2 -> 5 -> 3 -> 6 -> 4 -> 1 -> 7 -> NULL, K = 4
Output: 18

Approach: The idea is to use a sliding window of size k, keep track of sum of current window and update maximum sum if required. To implement sliding window two pointers can be used to represent starting and ending point. At each step first the value of node pointed by start is subtracted from current sum and the value of node pointed by end is added to current sum. This sum is compared to maximum sum and result is updated if required. The start and end pointers are incremented by one at each step.

Below is the implementation of above approach:

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
// Structure of a node
struct Node {
    int data;
    Node* next;
};
  
// Function to create new node
Node* newNode(int data)
{
    Node* node = new Node;
    node->next = NULL;
    node->data = data;
    return node;
}
  
// Function to return the maximum sum of
// k consecutive nodes
int findMaxSum(Node* head, int k)
{
    // To store current window sum
    int sum = 0;
  
    // To store maximum sum
    int maxSum = 0;
  
    // Pointer to the start of window
    Node* start = head;
  
    // Pointer to the end of window
    Node* end = head;
  
    int i;
  
    // Find the sum of first k nodes
    for (i = 0; i < k; i++) {
        sum += end->data;
        end = end->next;
    }
  
    maxSum = sum;
  
    // Move window by one step and
    // update sum. Node pointed by
    // start is excluded from current
    // window so subtract it. Node
    // pointed by end is added to
    // current window so add its value.
    while (end != NULL) {
  
        // Subtract the starting element
        // from previous window
        sum -= start->data;
        start = start->next;
  
        // Add the element next to the end
        // of previous window
        sum += end->data;
        end = end->next;
  
        // Update the maximum sum so far
        maxSum = max(maxSum, sum);
    }
  
    return maxSum;
}
  
// Driver code
int main()
{
    Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);
    head->next->next->next->next->next = newNode(6);
  
    int k = 5;
  
    cout << findMaxSum(head, k);
  
    return 0;
}

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Output:

20

Time Complexity: O(n)
Auxiliary Space: O(1)



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