static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
        SinglyLinkedListNode head = new SinglyLinkedListNode(-1);
        SinglyLinkedListNode curr = head;
        
        if (head1 == null) return head2;
        if (head2 == null) return head1;
        
        while (head1 != null && head2 != null) {
            if (head1.data < head2.data) {
                curr.next = head1;
                head1 = head1.next;
            } else {
                curr.next = head2;
                head2 = head2.next;
            }
            
            curr = curr.next;
        }
        
        if (head1 == null) {
            curr.next = head2;
        } else {
            curr.next = head1;
        }
        
        return head.next;
    }