# Maximum sum of K consecutive nodes in the given 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:

## C++

 `// C++ implementation of the approach` `#include ` `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;` `}`

## Java

 `// Java implementation of the approach` `class` `GFG` `{`   `// Structure of a node` `static` `class` `Node` `{` `    ``int` `data;` `    ``Node next;` `};`   `// Function to create new node` `static` `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` `static` `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 = Math.max(maxSum, sum);` `    ``}` `    ``return` `maxSum;` `}`   `// Driver code` `public` `static` `void` `main(String[] args)` `{` `    ``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``;` `    ``System.out.print(findMaxSum(head, k));` `}` `}`   `// This code is contributed by PrinciRaj1992`

## Python3

 `# Python3 implementation of the approach `   `# Node of Linked List` `class` `Node:` `    `  `    ``def` `__init__(``self``, x):` `        `  `        ``self``.data ``=` `x` `        ``self``.``next` `=` `None`   `# Function to return the maximum sum of` `# k consecutive nodes` `def` `findMaxSum(head, k):` `    `  `    ``# To store current window sum` `    ``sum` `=` `0`   `    ``# To store maximum sum` `    ``maxSum ``=` `0`   `    ``# Pointer to the start of window` `    ``start ``=` `head`   `    ``# Pointer to the end of window` `    ``end ``=` `head` `    `  `    ``# Find the sum of first k nodes` `    ``for` `i ``in` `range``(k):` `        ``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 !``=` `None``):`   `        ``# 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` `if` `__name__ ``=``=` `'__main__'``:` `    `  `    ``head ``=` `Node(``1``)` `    ``head.``next` `=` `Node(``2``)` `    ``head.``next``.``next` `=` `Node(``3``)` `    ``head.``next``.``next``.``next` `=` `Node(``4``)` `    ``head.``next``.``next``.``next``.``next` `=` `Node(``5``)` `    ``head.``next``.``next``.``next``.``next``.``next` `=` `Node(``6``)`   `    ``k ``=` `5`   `    ``print``(findMaxSum(head, k))`   `# This code is contributed by mohit kumar 29`

## C#

 `// C# implementation of the approach` `using` `System;`   `class` `GFG` `{`   `// Structure of a node` `public` `class` `Node` `{` `    ``public` `int` `data;` `    ``public` `Node next;` `};`   `// Function to create new node` `static` `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` `static` `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 = Math.Max(maxSum, sum);` `    ``}` `    ``return` `maxSum;` `}`   `// Driver code` `public` `static` `void` `Main(String[] args)` `{` `    ``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;` `    ``Console.Write(findMaxSum(head, k));` `}` `}`   `// This code is contributed by Rajput-Ji`

Output:

`20`

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

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