static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
    SinglyLinkedListNode dummy = new SinglyLinkedListNode(0);
    SinglyLinkedListNode curr = dummy;

    while (head1 != null && head2 != null) {
        if (head1.data > head2.data) {
            curr.next = head2;
            head2 = head2.next;
        }
        else {
            curr.next = head1;
            head1 = head1.next;
        }
        curr = curr.next;
    }

    // add left head2 nodes all
    if (head1 == null) curr.next = head2;

    // add left head1 nodes all
    if (head2 == null) curr.next = head1;

    return dummy.next;
}