简单实现1！+ 2！+3！...+n!算法

方法一：使用python内存方法reduce

from functools import reduce

def factorial(n):
    s = 0
    if not isinstance(n ,int):
        raise TypeError
    elif n in (0, 1):
        return 1
    else:
        for i in range(1, n+1):
            s = s + reduce(lambda x, y: x*y, range(1, i+1))
        return s

方法二：循环

def factorial(n):
   a = 0
   if not isinstance(n, int):
      raise TypeError
   elif n in (0, 1):
      return 1
   else:
      for i in range(1, n + 1):
         s = 1
         for j in range(1, i+1):
            s = s * j
         a = a + s
      return a