Odd length list
Input
head = [1,2,3,4,5]
Output
3
The middle of [1,2,3,4,5] is 3.
Full lesson preview
Find the middle node
Find the middle node of a singly linked list using the fast and slow pointer technique.
Python practice15 minLinked ListsIntermediateLast updated March 27, 2026
Problem statement
Given the head of a singly linked list, return the middle node. If there are two middle nodes (even length), return the second one. If the list is empty, return None. Aim to solve in O(n) time and O(1) extra space using the two-pointer technique.
Task
Return the middle node of a singly linked list. If the list has an even number of nodes, return the second middle node.
Examples
Even length list
Input
head = [1,2,3,4]
Output
3
For even length, return the second middle (3).
Input format
A linked list represented by its head node. Use create_linked_list([...]) to build inputs.
Output format
Return the middle node (ListNode). Tests will often access the .val of the returned node. Return None for an empty list.
Constraints
- The list length can be 0 or more.
- Use O(1) extra space.
- Time complexity should be O(n).
Samples
Sample 1
Input
[10, 20, 30, 40, 50]
Output
30
The middle node's value is 30.