# 循环计算阶乘函数

0,1,2,3,4,5,6,7,8,9,10! 令人惊讶的是，6个星期的秒数居然也等于10!

不使用函数递归，实现一个阶乘计算函数(n<=170)：

```python
def fact(n):
    r = 1
    # TODO(You): 请在此编写代码
    return r


if __name__ == '__main__':
    print(fact(10))
```

请选出下面的 Python 非递归阶乘实现代码中，<span style="color:red">错误</span> 的选项。

## template

```python
def fact1(n):
    r = 1
    for i in range(0, n):
        r *= (i+1)
    return r


def fact2(n):
    r = 1
    while (n > 0):
        r *= n
        n -= 1
    return r


def fact3(op):
    import math
    z = op + 1
    p = [1.000000000190015, 76.18009172947146, -86.50532032941677,
         24.01409824083091, -1.231739572450155, 1.208650973866179E-3, -5.395239384953E-6]

    d1 = math.sqrt(2 * math.pi) / z

    i = 1
    d2 = p[0]
    while i <= 6:
        d2 += p[i] / (z + i)
        i += 1

    d3d4 = math.pow((z + 5.5), (z + 0.5))*math.exp(-(z + 5.5))
    d = d1 * d2 * d3d4
    return int(d)


def fact4(n):
    import math
    r = math.factorial(n)
    return r


if __name__ == '__main__':
    print(fact1(10))
    print(fact2(24))
    print(fact3(24))
    print(fact4(10))
```

## 答案

```python
import math
z = n + 1
p = [1.000000000190015, 76.18009172947146, -86.50532032941677,
        24.01409824083091, -1.231739572450155, 1.208650973866179E-3, -5.395239384953E-6]

d1 = math.sqrt(2 * math.pi) / z

i = 1
d2 = p[0]
while i <= 6:
    d2 += p[i] / (z + i)
    i += 1

d3d4 = math.pow((z + 5.5), (z + 0.5))*math.exp(-(z + 5.5))
d = d1 * d2 * d3d4
r = int(d)
```

## 选项

### A

```python
for i in range(0, n):
    r *= (i+1)
```

### B

```python
while n > 0:
    r *= n
    n -= 1
```

### C

```python
import math
r = math.factorial(n)
```