sys.setrecursionlimit(2000)

def mergeLists(head1, head2):
    if not (head1 and head2):
        return head1 if head1 else head2

    if head1.data < head2.data:
        head1.next = mergeLists(head1.next, head2)
        return head1
    else:
        head2.next = mergeLists(head1, head2.next)
        return head2