9 min, including test
import math
def nthperm(li, n):
n -= 1
s = len(li)
res = []
if math.factorial(s) <= n:
return None
for x in range(s-1,-1,-1):
f = math.factorial(x)
d = n / f
n -= d * f
res.append(li[d])
del(li[d])
return res
#now that's fast...
nthperm(range(40), 123456789012345678901234567890)