python 阶乘(n!)

使用for循环实现阶乘

a = int(input('please input a interger: '))
num = 1
if a < 0:
    print('负数没有阶乘！')
elif a == 0:
    print('0的阶乘是1')
else:
    for i in range(1,a+1):
        num *= i
    print(num)