阶乘最后非零位

#include<string.h>
#define MAXN  10000
int lastdigit(char*buf)
{
const int mod[20] = {1,1,2,6,4,2,2,4,2,8,4,4,8,4,6,8,8,6,8,2};
int len = strlen(buf),a[MAXN],i,c,ret = 1;
if(len == 1)
return mod[buf[0]-'0'];
for(i = 0;i<len;i++)
a[i] = buf[len-1-i]-'0';
for(;len;len-=!a[len-1])
{
ret = ret*mod[a[1]%2*10+a[0]]%5;
for(c = 0,i = len-1;i>=0;i--){
c = c*10+a[i];
a[i] = c/5;
c%=5;
}
}
return ret+ret%2*5;
}