Last non-zero Digit in N! http://acm.hdu.edu.cn/showproblem.php?pid=1066

 1 #include<cstdio> 2 #include<cstring> 3 const int M=1024; 4 const int tomod[20]= {1,1,2,6,4,2,2,4,2,8,4,4,8,4,6,8,8,6,8,2}; 5 int lastdigit(char buf[]){//阶乘最后非0位O(nlogn)返回该位,n以字符串方式传入 6     int len=strlen(buf),ret=1,a[M]; 7     if(len==1) return tomod[buf[0]-‘0‘]; 8     for(int i=0;i<len;i++) a[i]=buf[len-1-i]-‘0‘; 9     for(;len;len-=!a[len-1]){10         ret=ret*tomod[a[1]%2*10+a[0]]%5;11         for(int c=0,i=len-1;i>=0;i--){12             c=c*10+a[i];13             a[i]=c/5;14             c%=5;15         }16     }17     return ret+ret%2*5;18 }19 char str[M];20 int main() {21     while(~scanf("%s",str)) {22         printf("%d\n",lastdigit(str));23     }24     return 0;25 }