In [3]: factorial?

Signature: factorial(x, /)

Docstring:

Find x!.

Raise a ValueError if x is negative or non-integral.

Type:      builtin_function_or_method

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Python 3.8以上版本才有的功能，ZJ是3.6所以不能直接用

In [4]: comb?

Signature: comb(n, k, /)

Docstring:

Number of ways to choose k items from n items without repetition and without order.

Evaluates to n! / (k! * (n - k)!) when k <= n and evaluates

to zero when k > n.

Also called the binomial coefficient because it is equivalent

to the coefficient of k-th term in polynomial expansion of the

expression (1 + x)**n.

Raises TypeError if either of the arguments are not integers.

Raises ValueError if either of the arguments are negative.

Type:      builtin_function_or_method

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如果要自己幹，也可以試試functools.reduce

系統升到 python 3.8 了

現在 python 可以使用 math.comb 寫這題，會輕鬆很多 (之前的版本比較舊，沒有這功能)