hash poj_1200 Crazy Search

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
using namespace std;
int n,nc;
char s[1000000];
int _hash[16000005],num[257];
int main()
{
//    freopen("in","r",stdin);
while(scanf("%d%d",&n,&nc)!=EOF)
{
memset(_hash,0,sizeof(_hash));
memset(num,0,sizeof(num));
memset(s,'\0',sizeof(s));
scanf("%s",s);
int len=strlen(s);
int seed=0;
//        printf("here\n");
for(int i=0;i!=len;i++)
{
if(!num[s[i]])
{
num[s[i]]=++seed;
}
if(seed==nc)
{
break;
}
}
//        printf("here\n");
int sum=0;
int ans=0;
for(int i=0;i<=len-n;i++)
{
sum=0;
for(int j=0;j<n;j++)
{
sum=sum*nc+num[s[i+j]]-1;
}
if(!_hash[sum])
{
_hash[sum]=1;
ans++;
}
}
printf("%d\n",ans);
}
return 0;
}