fastest way to merge lists.

[Roy Smith]

I've got 5 (unsorted) lists of strings.  Each list has O(10^4) strings 
in it.  Within each list, each string appears exactly once.  Also, there 
is a large overlap between the lists.

I want to form a single list which contains exactly one copy of each 
string from the combined lists.  In unix, this would be:

cat list[12345] | sort | uniq

I'm not yet sure if care if the final list is sorted or not.  For now, I 
think it doesn't matter.  What's the fastest way to do that?  It seems 
to me:

d = {}
for s in list1:
   d[s] = 1
for s in list2:
   d[s] = 1
for s in list3:
   d[s] = 1
for s in list4:
   d[s] = 1
for s in list5:
   d[s] = 1
list = d.keys()
del d   # if I care about memory

would be pretty good.  Anything better?

Now, if I want it sorted, I could just sort the above list.

I suspect that if I knew ahead of time I wanted the final list sorted, 
sorting each individual list then doing a linear merge might make more 
sense, if there was little overlap between the lists.  But, since I know 
they overlap greatly (i.e. len(list) will be approximately the same as 
len(listN)), I'm guessing doing it as described above and sorting the 
result might just work out faster.