Flattening a Linked List

class Node {
    int data;
    Node next;
    Node bottom;

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

class Solution {
    public Node flatten(Node node) {
        if (node == null || node.next == null)
            return node;
        Node temp = node;
        while (temp.next.next != null)
            temp = temp.next;
        // merge the last node and bottom of previous node
        Node mergedList = mergeList(temp.bottom, temp.next);

        temp.bottom = mergedList;
        temp.next = null;
        return flatten(node);
    }

    public Node mergeList(Node head1, Node head2) {
        if (head1 == null)
            return head2;
        if (head2 == null)
            return head1;

        Node dummy = new Node(-1);
        Node curr = dummy;
        Node ptr1 = head1;
        Node ptr2 = head2;
        while (ptr1 != null && ptr2 != null) {
            if (ptr1.data < ptr2.data) {
                curr.bottom = ptr1;
                ptr1 = ptr1.bottom;

            } else {
                curr.bottom = ptr2;
                ptr2 = ptr2.bottom;
            }
            curr = curr.bottom;
        }
        if (ptr1 != null)
            curr.bottom = ptr1;
        else
            curr.bottom = ptr2;

        return dummy.bottom;
    }
}