Modify contents of Linked List – Recursive approach

Given a singly linked list containing n nodes. Modify the value of first half nodes such that 1st node’s new value is equal to the last node’s value minus first node’s current value, 2nd node’s new value is equal to the second last node’s value minus 2nd node’s current value, likewise for first half nodes. If n is odd then the value of the middle node remains unchanged.

Examples:

Input: 10 -> 4 -> 5 -> 3 -> 6
Output: -4 -> -1 -> 5 -> 3 -> 6

Input: 2 -> 9 -> 8 -> 12 -> 7 -> 10
Output: 8 -> -2 -> 4 -> 12 -> 7 -> 10

Approach: Traverse the link list using recursion for only second half of the list rather than recursing the whole list, thereby reducing the stack frame. Based on the number of nodes in the list, we calculate the starting point for the second half of the list and the list is recursed for the second half. Once the second half is recursed completely, the value of the node present in the stack and the value of the current node is subtracted.

Below is the implementation of the above approach:

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// C++ program to modify the contents
// of the linked list with recursion
  
#include <bits/stdc++.h>
using namespace std;
  
// Represents a Node of the linked list
struct Node {
    int data;
    Node* next;
};
  
// Function to create and return a new node
Node* insert(int data)
{
    Node* temp;
    temp = new Node;
    temp->data = data;
    temp->next = NULL;
    return temp;
}
  
// Function which returns the total number of 
// node present in the list
int getTotalNodeCount(Node* head)
{
    int totalNodesCount = 0;
      
    while(head != NULL)
    {
        totalNodesCount++;
        head = head->next;
    }
    return totalNodesCount;
}
  
// Function to modify the contents
// of the given linked lists
void modifyContents(Node** first_half , 
                        Node* second_half) 
{
    if (second_half == NULL)
    {
        return;
    }
      
    modifyContents(first_half,second_half->next);
      
    (*first_half)->data = second_half->data - (*first_half)->data;
    (*first_half) = (*first_half)->next;
      
    return;
}
  
// Wrapper function which calculates the starting 
// point for second half of the list
void modifyContentsWrapper(Node** head)
{
    Node *ptr = NULL; // pointer to second half of list
    Node *temp = NULL; 
    int diff = 0; 
    int length = 0; // number of nodes in the list
      
    if (*head == NULL)
    {
        return;
    }
    length = getTotalNodeCount(*head);
      
    // If link list has odd number of nodes, then pointer 
    // for second half starts from (length of link list + 1).
    // Say for an example, for the list 10->4->5->3->6 pointer 
    // should start from 3.
    // If list has even number of nodes, 10->4->5->9->6->8, 
    // pointer starts from 9.
    diff = (length%2 == 0? (length/2) :(length/2)+1 );
      
    ptr = *head;
  
    while(diff--)
        ptr = ptr->next;
  
    temp = *head;
  
    modifyContents(&temp,ptr);
  
    return;
}
  
// Function to print the contents of the linked list
void print(Node* nod)
{
    if (nod == NULL) {
        return;
    }
      
    cout << nod->data;
      
    if (nod->next != NULL)
        cout << " -> ";
      
    print(nod->next);
}
  
// Driver code
int main()
{
    Node* head = insert(2);
    head->next = insert(9);
    head->next->next = insert(8);
    head->next->next->next = insert(12);
    head->next->next->next->next = insert(7);
    head->next->next->next->next->next = insert(10);
      
    // Modify the linked list
    modifyContentsWrapper(&head);
  
    // Print the modified linked list
    print(head);
      
    return 0;
}

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Output:

8 -> -2 -> 4 -> 12 -> 7 -> 10


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