Find Middle Node Of A Linked List Problem Statement

Given a linked list, find its middle element.

If the list has even number of nodes, return the second of the two middle nodes.

Example One

{
"head": [1, 2, 3, 4, 5]
}

Output:

3

Example Two

{
"head": [1, 2, 3, 4]
}

Output:

3

Notes

Constraints:

• 0 <= number of nodes <= 10⁵
• -2 * 10⁹ <= node value <= 2 * 10⁹
• Do it in one pass over the list
• If given linked list is empty, return null.

We have provided one solution for this problem.

Find Middle Node Of A Linked List Solution: Optimal

A naive solution involves computing the length of the list and then traversing exactly half of that.

A faster approach is to use a slow pointer and a fast pointer, with the faster one traversing twice as fast.

By the time the fast one reaches the end of the list, the slower one is at the midpoint. This solution still takes O(n) time but it is faster than the naive one.

Time Complexity

O(n).

Auxiliary Space Used

O(1).

Space Complexity

O(n).

Code For Find Middle Node Of A Linked List Solution: Optimal

/*
* Asymptotic complexity in terms of length of the given linked list `n`:
* Time: O(n).
* Auxiliary space: O(1).
* Total space: O(n).
*/

LinkedListNode* find_middle_node(LinkedListNode* head)
{
    LinkedListNode *slow_ptr = head;
    LinkedListNode *fast_ptr = head;
    while (fast_ptr && fast_ptr->next)
    {
        // Forward two steps.
        fast_ptr = fast_ptr->next->next;
        // Forward one step.
        slow_ptr = slow_ptr->next;
    }
    return slow_ptr;
}

We hope that these solutions to finding the middle node in a singly linked list problem have helped you level up your coding skills. You can expect problems like these at top tech companies like Amazon and Google.

If you are preparing for a tech interview at FAANG or any other Tier-1 tech company, register for Interview Kickstart’s FREE webinar to understand the best way to prepare.

Interview Kickstart offers interview preparation courses taught by FAANG+ tech leads and seasoned hiring managers. Our programs include a comprehensive curriculum, unmatched teaching methods, and career coaching to help you nail your next tech interview.

We offer 18 interview preparation courses, each tailored to a specific engineering domain or role, including the most in-demand and highest-paying domains and roles, such as:

• Back-end Engineering Interview Course
• Front-end Engineering Interview Course
• Full Stack Developer Interview Course

‍To learn more, register for the FREE webinar.