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Code Single linked list

Insert node at middle of linked list

The given problem involves inserting a new node into the middle of a singly linked list. A linked list is a data structure where elements, called nodes, are connected sequentially. Each node contains data and a reference to the next node in the sequence. The objective is to insert a new node with a given value at the middle position of the linked list.

Problem Statement

Given a singly linked list, we need to insert a new node containing a given value at the middle position of the list. If the list has an odd number of nodes, the middle position will be exactly in the center. If the list has an even number of nodes, we consider the middle position to be slightly towards the end of the first half of the list.

Description with Example

Consider the following example to illustrate the problem:

Original Linked List: 1 → 2 → 3 → 4 → 5

We want to insert the value 6 at the middle position.

After Insertion: 1 → 2 → 3 → 6 → 4 → 5

Idea to Solve the Problem

insert linked list node in middle position

To solve this problem, we can use a two-pointer approach. We'll maintain two pointers: one will move at twice the speed of the other. When the fast-moving pointer reaches the end of the list, the slow-moving pointer will be at the middle. Then, we can insert the new node after the slow-moving pointer.


AddNodeAtMiddle(sll, data):
    Create a new node with 'data'
    If sll.head is NULL:
        Set sll.head to the new node
        Initialize two pointers: 'slow' and 'fast' as sll.head
        While fast is not NULL and is not NULL:
            Move fast to
            Move slow to
        Set new node's next to
        Set to the new node

Algorithm Explanation

  1. Create a new node with the given data.
  2. If the linked list is empty (sll.head is NULL), set sll.head to the new node and return.
  3. Initialize two pointers 'slow' and 'fast' to the head of the linked list.
  4. Traverse the linked list with the following condition:
    • Move 'fast' two steps at a time (fast =
    • Move 'slow' one step at a time (slow =
  5. When the 'fast' pointer reaches the end (fast is NULL or is NULL), the 'slow' pointer will be at the middle position.
  6. Set the new node's next pointer to ''.
  7. Set '' to the new node.
  8. The new node has been inserted at the middle of the linked list.

Time Complexity

The time complexity of this algorithm is O(n), where n is the number of nodes in the linked list. The algorithm traverses the entire list once to find the middle position and then performs constant-time operations to insert the new node. Therefore, the overall time complexity is linear in terms of the number of nodes in the linked list.


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