Java Program For Swapping Kth Node From Beginning With Kth Node From End In A Linked List

// A Java program to swap kth
// node from the beginning with
// kth node from the end

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

class LinkedList 
{
    Node head;

    /* Utility function to insert 
       a node at the beginning */
    void push(int new_data)
    {
        Node new_node = 
             new Node(new_data);
        new_node.next = head;
        head = new_node;
    }

    /* Utility function for displaying 
       linked list */
    void printList()
    {
        Node node = head;
        while (node != null) 
        {
            System.out.print(node.data + " ");
            node = node.next;
        }
        System.out.println("");
    }

    /* Utility function for calculating 
       length of linked list */
    int countNodes()
    {
        int count = 0;
        Node s = head;
        while (s != null) {
            count++;
            s = s.next;
        }
        return count;
    }

    /* Function for swapping kth nodes from 
       both ends of linked list */
    void swapKth(int k)
    {
        // Count nodes in linked list
        int n = countNodes();

        // Check if k is valid
        if (n < k)
            return;

        // If x (kth node from start) and
        // y(kth node from end) are same
        if (2 * k - 1 == n)
            return;

        // Find the kth node from beginning of 
        // linked list. We also find previous 
        // of kth node because we need to update 
        // next pointer of the previous.
        Node x = head;
        Node x_prev = null;
        for (int i = 1; i < k; i++) 
        {
            x_prev = x;
            x = x.next;
        }

        // Similarly, find the kth node from end 
        // and its previous. kth node from end 
        // is (n-k+1)th node from beginning
        Node y = head;
        Node y_prev = null;
        for (int i = 1; i < n - k + 1; i++) 
        {
            y_prev = y;
            y = y.next;
        }

        // If x_prev exists, then new next of it 
        // will be y. Consider the case when y->next 
        // is x, in this case, x_prev and y are same. 
        // So the statement "x_prev->next = y" creates 
        // a self loop. This self loop will be broken 
        // when we change y->next.
        if (x_prev != null)
            x_prev.next = y;

        // Same thing applies to y_prev
        if (y_prev != null)
            y_prev.next = x;

        // Swap next pointers of x and y. These 
        // statements also break self loop if 
        // x->next is y or y->next is x
        Node temp = x.next;
        x.next = y.next;
        y.next = temp;

        // Change head pointers when k is 1 or n
        if (k == 1)
            head = y;

        if (k == n)
            head = x;
    }

    // Driver code 
    public static void main(String[] args)
    {
        LinkedList llist = 
                   new LinkedList();
        for (int i = 8; i >= 1; i--)
            llist.push(i);

        System.out.print(
        "Original linked list: ");
        llist.printList();
        System.out.println("");

        for (int i = 1; i < 9; i++) 
        {
            llist.swapKth(i);
            System.out.println(
            "Modified List for k = " + i);
            llist.printList();
            System.out.println("");
        }
    }
}