To count number of quadruplets with sum = 0

Paul Rubin http
Fri Mar 16 06:59:52 CET 2007

"n00m" <n00m at> writes:
> h = collections.defaultdict(itertools.repeat(0).next)

Something wrong with 
   h = collections.defaultdict(int)

> for x in e:
>   for y in r:
>     sch += h[-(x + y)]

That scares me a little: I think it makes a new entry in h, for the
cases where -(x+y) is not already in h.  You want:

     for x in e:
       for y in r:
        sch += h.get(-(x+y), 0)

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