Project Euler Problem 20 Solved

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!

Solution

1
2
3
4
5
6
7
8
#!/usr/bin/python
# -*- coding: utf-8 -*-

if __name__ == '__main__':
    product = 1
    for i in range(2, 101):
        product *= i
    print sum([int(x) for x in str(product)])

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
package main

import (
    "fmt"
    "math/big"
    "strconv"
)

func main() {
    product := big.NewInt(1)
    for i := 1; i < 101; i++ {
        product.Mul(product, big.NewInt(int64(i)))
    }
    sum := 0
    for idx := range product.String() {
        tmp, _ := strconv.Atoi(product.String()[idx : idx+1])
        sum += tmp
    }
    fmt.Println(sum)
}

I’m the 100089th person to have solved this problem.