Merge Sort for Linked Lists

// C# program for merge sort on singly linked list

using System;

class Node {
    public int data;
    public Node next;
    public Node(int x)
    {
        data = x;
        next = null;
    }
}

// Function to split the singly linked list into two halves
class GfG {
    static Node Split(Node head)
    {
        Node fast = head;
        Node slow = head;

        // Move fast pointer two steps and slow pointer
        // one step until fast reaches the end
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            if (fast != null) {
                slow = slow.next;
            }
        }

        // Split the list into two halves
        Node temp = slow.next;
        slow.next = null;
        return temp;
    }

    // Function to merge two sorted singly linked lists
    static Node Merge(Node first, Node second)
    {

        // If either list is empty, return the other list
        if (first == null)
            return second;
        if (second == null)
            return first;

        // Pick the smaller value between first and second
        // nodes
        if (first.data < second.data) {

            // Recursively merge the rest of the lists and
            // link the result to the current node
            first.next = Merge(first.next, second);
            return first;
        }
        else {

            // Recursively merge the rest of the lists
            // and link the result to the current node
            second.next = Merge(first, second.next);
            return second;
        }
    }

    // Function to perform merge sort on a singly linked
    // list
    static Node MergeSort(Node head)
    {

        // Base case: if the list is empty or has only one
        // node, it's already sorted
        if (head == null || head.next == null)
            return head;

        // Split the list into two halves
        Node second = Split(head);

        // Recursively sort each half
        head = MergeSort(head);
        second = MergeSort(second);

        // Merge the two sorted halves
        return Merge(head, second);
    }

    static void PrintList(Node head)
    {
        Node curr = head;
        while (curr != null) {
            Console.Write(curr.data + " ");
            curr = curr.next;
        }
        Console.WriteLine();
    }

    public static void Main()
    {
        // Create a hard-coded singly linked list:
        // 9 -> 8 -> 5 -> 2
        Node head = new Node(9);
        head.next = new Node(8);
        head.next.next = new Node(5);
        head.next.next.next = new Node(2);

        head = MergeSort(head);
        PrintList(head);
    }
}