Is there such an idiom?

[Per]

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?