def mergeLists(head1, head2):
    ptr = ret = opposite = None
    if head1.data < head2.data:
        ptr = head1
        ret = head1
        opposite = head2
    else:
        ptr = head2
        ret = head2
        opposite = head1
    
    while ptr:
        while ptr.next and ptr.next.data < opposite.data:
            ptr = ptr.next
        
        next_p = ptr.next
        ptr.next = opposite
        ptr = opposite
        opposite = next_p

        if opposite is None:
            return ret