Last non-zero Digit in N!

 1 #include <iostream>
 2 #include <string.h>
 3 #include <stdio.h>
 4 #include <stdlib.h>
 5 #define MAXN 10000
 6 
 7 int lastdigit(char* buf){
 8     const int mod[20]={1,1,2,6,4,2,2,4,2,8,4,4,8,4,6,8,8,6,8,2};
 9     int len=strlen(buf),a[MAXN],i,c,ret=1;
10     if (len==1)
11         return mod[buf[0]-'0'];
12     for (i=0;i<len;i++)
13         a[i]=buf[len-1-i]-'0';
14     for (;len;len-=!a[len-1]){
15         ret=ret*mod[a[1]%2*10+a[0]]%5;
16         for (c=0,i=len-1;i>=0;i--)
17             c=c*10+a[i],a[i]=c/5,c%=5;
18     }
19     return ret+ret%2*5;
20 }
21 int main()
22 {
23     char a[1000]="\0";
24     while(scanf("%s",a)!=EOF)
25     {
26         int ans=lastdigit(a);
27         printf("%d\n",ans);
28     }
29     return 0;
30 }