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# Factorial of a number using memoization
result = [-1] * 10
result[0] = result[1] = 1
def factorial(num):
"""
>>> factorial(7)
5040
>>> factorial(-1)
'Number should not be negative.'
>>> [factorial(i) for i in range(5)]
[1, 1, 2, 6, 24]
"""
if num < 0:
return "Number should not be negative."
if result[num] != -1:
return result[num]
else:
result[num] = num * factorial(num - 1)
# uncomment the following to see how recalculations are avoided
# print(result)
return result[num]
# factorial of num
# uncomment the following to see how recalculations are avoided
##result=[-1]*10
##result[0]=result[1]=1
##print(factorial(5))
# print(factorial(3))
# print(factorial(7))
if __name__ == "__main__":
import doctest
doctest.testmod()