基础练习 阶乘计算

#include <cstdio>
#include <cstring>
#include <cmath>
#define PI 3.141592654
#define E 2.71828182846
#define MAX 30000

int Len(int n)
{
    int s = 1;
    if(n > 3)
        s = log10(2 * PI * n) / 2 + n * log10( n / E) + 1;
    return s;
}

int main()
{
	int a[MAX], i, j, n;
	int c = 0;                                   //进位
	int s;
	int len;
	memset(a, 0, sizeof(a));                     //初始化字串 
	scanf("%d", &n);
	a[0] = 1;
	for (i = 2; i <= n; i++)                     //乘数
	{
		len = Len(i);
		for (j = 0; j<len; j++)
		{
			s = a[j] * i + c;
			a[j] = s % 10;
			c = s / 10;
		}
	}
	for (j = Len(n) - 1; j >= 0; j--)
	{
		printf("%d", a[j]);
	}
	return 0;
}