Middle of the Linked List in Java

Middle of the Linked List in Java

This guide explains the intuition, optimized approach, and Java implementation for middle of the linked list in java, with practical tips for interviews and production coding standards.

Problem

Return middle node of a linked list. If two middles exist, return second one.

Approach

• slow moves 1 step
• fast moves 2 steps

When fast reaches end, slow is at middle.

Java Solution

class Solution {
    public ListNode middleNode(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;

        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }

        return slow;
    }
}

Complexity

• Time: O(n)
• Space: O(1)

Note

This same pointer pattern is reused in cycle detection and palindrome linked list checks.

Key Takeaways

• Start from the brute-force idea, then derive the optimized invariant.
• Use clean pointer/index boundaries to avoid off-by-one bugs.
• Validate against edge cases (empty input, single element, duplicates, extreme values).