def JieCheng(n):
sum=1
for i in range(1,n+1):
sum*=i
return sum
print JieCheng(40)/JieCheng(20)/JieCheng(20)
相当于从40中选出20来。C40->20
def JieCheng(n):
sum=1
for i in range(1,n+1):
sum*=i
return sum
print JieCheng(40)/JieCheng(20)/JieCheng(20)
相当于从40中选出20来。C40->20

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