Three methods of factorial python

# python三种方法实现阶乘
# while循环实现阶乘
def fact(n):
    if n == 0:
        return 1  # 0 的阶乘为 1
    elif n < 0:
        return '负数没有阶乘'
    s = 1
    while n:
        s = s*n
        n -= 1
    return s
print(fact(-3))

#递归实现阶乘
def fact2(n):
    if n == 0:
        return 1    # 0 的阶乘为 1
    elif n < 0 :
        return '负数没有阶乘'
    elif n == 1:
        return 1
    else:
        return n*fact2(n-1)
r = fact2(4)
print(r)

#range实现阶乘
def fact3(n):
    if n == 0:
        return 1     # 0 的阶乘为 1
    elif n < 0:
        return '负数没有阶乘'
    else:
        s = 1
        for i in range(1,n+1):
            s = s*i
        return s
r = fact3(0)
print(r)