876. Middle of the Linked List

At a Glance

• Topic: Linked List
• Pattern: Analyze Pattern
• Difficulty: Easy
• LeetCode: 876

Problem Statement

Given the head of a singly linked list, return the middle node of the linked list.

If there are two middle nodes, return the second middle node.

Example 1:

Input: head = [1,2,3,4,5] Output: [3,4,5] Explanation: The middle node of the list is node 3.

Example 2:

Input: head = [1,2,3,4,5,6] Output: [4,5,6] Explanation: Since the list has two middle nodes with values 3 and 4, we return the second one.

Constraints:

The number of nodes in the list is in the range [1, 100].
1 <= Node.val <= 100

Approach & Solution Steps

Use slow and fast pointers. Advance slow by one step and fast by two steps. When fast reaches the end, slow will be at the middle.

Optimal JS Solution

function middleNode(head) {
  let slow = head, fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
  }
  return slow;
}

Edge Cases & Pitfalls

• Always consider empty or null inputs.
• Watch out for off-by-one index errors.