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Project Euler: Problem 20: Factorial digit sum
n! means n × (n ? 1) × ... × 3 × 2 × 1
For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
and the sum of the digits in the number 10! is
3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
Find the sum of the digits in the number 100!
Answer: 648
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n = 100
fac = 1  #初始化阶乘结果
while n >= 1:
    fac *= n
    n -= 1

# 提取出阶乘结果的每个数字，形成列表lst
lst = [int(i) for i in str(fac)]

res = 0  #初始化相加结果
for i in range(len(lst)):
    res += lst[i]
print(res)