Middle of the Linked List

Link: https://leetcode.com/problems/middle-of-the-linked-list/

Constraints:

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

Idea

Use two pointers. Slow pointer advance one node at a time, while fast pointer advances two nodes at a time. When the fast pointer reaches the end, slow pointer is pointing to the middle node.

Code

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;
    }
}

• Time: O(n).
• Space: O(1).