python 階乘(n!)

使用for循環實現階乘

a = int(input('please input a interger: '))
num = 1
if a < 0:
    print('負數沒有階乘！')
elif a == 0:
    print('0的階乘是1')
else:
    for i in range(1,a+1):
        num *= i
    print(num)