这段代码能优化下吗？，很占内存，如果磁盘不灵甚至就会死机

Question

```python
from itertools import permutations
fac=lambda x: len(list(permutations({i for i in range(x)}) ))
for t in range(50):
    e=sum(1/fac(x) for x in range(t))
    print(e)

```

Answer 1

fac是求全排列个数,用阶乘就好了

from math import factorial


for t in range(50):
    e=sum(1/factorial(x) for x in range(t))
    print(e)