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!
题目大意：
n! = n × (n − 1) × … × 3 × 2 × 1
例如，10! = 10 × 9 × … × 3 × 2 × 1 = 3628800, 那么10!的各位之和就是3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
算出100!的各位之和。
#include <stdio.h>
#include <math.h>
#define N 100
int main(void)
{
int n = N * log(N / 3), a[n], ca = 0, i, j;
for (i = 0; i < n; i++)
a[i] = 0;
a[n - 1] = 1;
for (i = 1; i <= N; i++) {
for (j = n - 1; j >= 0; j--) {
ca = i * a[j] + ca;
a[j] = ca % 10;
ca /= 10;
}
ca = 0;
}
ca = 0;
for (i = 0; i < n; i++)
ca += a[i];
printf("%d\n", ca);
return 0;
}
Answer:648
Completed on Sun, 13 Apr 2014, 02:47
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