Is there such an idiom?

[Ron Adam]

Per wrote:
> http://jaynes.colorado.edu/PythonIdioms.html
> 
> """Use dictionaries for searching, not lists. To find items in common
> between two lists, make the first into a dictionary and then look for
> items in the second in it. Searching a list for an item is linear-time,
> while searching a dict for an item is constant time. This can often let
> you reduce search time from quadratic to linear."""
> 
> Is this correct?
> s = [1,2,3,4,5...]
> t = [4,5,6,,8,...]
> how to find whether there is/are common item(s) between two list in
> linear-time?
> how to find the number of common items between two list in linear-time?

I'm not sure if it works in linear time, but if there are no duplicates 
in each list, sets would be the easiest way to do these with.

>>> s = set([1,2,3,4,5,6])
>>> t = set([4,5,6,7,8,9])
>>> s.intersection(t)
set([4, 5, 6])
>>> len(s.intersection(t))
3

Cheers,
    Ron