Find kth node from Middle towards Head of a Linked List in C++

In this problem, we are given a linked-list and a number k. Our task is to find kth node from Middle towards the Head of a Linked List. 

Let’s take an example to understand the problem, 

Input: linked-list : 4 -> 2 -> 7 -> 1 -> 9 -> 12 -> 8 -> 10 -> 5, k = 2

Output: 7

Explanation: 

Middle node value is 9.

The 2nd node from middle towards head is 7.

Solution Approach

We need to find kth element from the middle of the linked-list towards the beginning. For this we need to find the size of linked-list by traversing the linked-list from beginning to end and find size.

K element from the middle towards the start is the (n/2 + 1 - k)th element from the beginning.

Program to illustrate the working of our solution,

Example

Live Demo

#include <iostream>
using namespace std;

struct Node {
   int data;
   struct Node* next;
};

void pushNode(struct Node** head_ref, int new_data)
{
   struct Node* new_node = new Node;
   new_node->data = new_data;
   new_node->next = (*head_ref);
   (*head_ref) = new_node;
}

int findKmiddleNode(struct Node* head_ref, int k) {
   
   int n = 0;
   struct Node* counter = head_ref;
   while (counter != NULL) {
      n++;
      counter = counter->next;
   }
   int reqNode = ((n / 2 + 1) - k);

   if (reqNode <= 0)
      return -1;
     
   struct Node* current = head_ref;
   int count = 1;
   while (current != NULL) {
      if (count == reqNode)
         return (current->data);
      count++;
      current = current->next;
   }
}

int main()
{

   struct Node* head = NULL;
   int k = 2;
   pushNode(&head, 5);
   pushNode(&head, 10);
   pushNode(&head, 8);
   pushNode(&head, 12);
   pushNode(&head, 9);
   pushNode(&head, 1);
   pushNode(&head, 7);  
   pushNode(&head, 2);
   pushNode(&head, 4);

   cout<<k<<"th element from beginning towards head is "<<findKmiddleNode(head, k);

   return 0;
}

Output

2th element from beginning towards head is 7