def mergeLists(head1, head2):
    dummy = SinglyLinkedListNode(0)         # A dummy node to start the merged list
    tail = dummy            # Tail is where we add new nodes

    while head1 and head2:
        if head1.data <= head2.data:
            tail.next = head1
            head1 = head1.next
        else:
            tail.next = head2
            head2 = head2.next
        tail = tail.next    # Move tail forward

    # Attach the rest of the non-empty list
    if head1:
        tail.next = head1
    elif head2:
        tail.next = head2

    return dummy.next