Middle of the Linked List - LeetCode Daily Challenge

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.

Try here before watching the video.

Video Explanation

Java Code

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

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

        return slow;
    }
}

C++ 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;
    }
};

Python Code

class Solution:
    def middleNode(self, head: Optional[ListNode]) -> Optional[ListNode]:
        slow = head
        fast = head

        while fast is not None and fast.next is not None:
            slow = slow.next
            fast = fast.next.next

        return slow

Javascript Code

var middleNode = function(head) {
    let slow = head;
    let fast = head;
    
    while (fast !== null && fast.next !== null) {
        slow = slow.next;
        fast = fast.next.next;
    }

    return slow;
};

Go Code

func middleNode(head *ListNode) *ListNode {
    slow := head
    fast := head
    
    for fast != nil && fast.Next != nil {
        slow = slow.Next
        fast = fast.Next.Next
    }
    
    return slow
}

Complexity Analysis

Time Complexity: O(N), where N is the number of Nodes in the linked list.
Space Complexity: O(1), we are not taking any extra space.