def mergeLists(head1, head2):
    if not head1 and not head2:
        return head1
    if not head1:
        return head2
    if not head2:
        return head1
    res = SinglyLinkedListNode(None)
    c = res
    c1 = head1
    c2 = head2
    while c1 and c2:
        print(res)
        if c1.data <= c2.data:
            c.next = c1
            c1 = c1.next
        else:
            c.next = c2
            c2 = c2.next
        c = c.next
    if c1:
        c.next = c1
    if c2:
        c.next = c2
    return res.next