hdu 3336 Count the string

#include <cstring>
#include <cstdio>
#define Mod 10007
#define N 200005
int L,T,nextt[N],sum,f[N];
char str[N];
void get_next()
{
    int i=0,j=-1,len=strlen(str);
    nextt[i]=j;
    for(;i<len;)
    {
        if(j==-1||str[i]==str[j]) i++,j++,nextt[i]=j;
        else j=nextt[j];
    }
}
int main()
{
    scanf("%d",&T);
    for(;T--;)
    {
        memset(f,0,sizeof(f));sum=0;
        scanf("%d",&L);
        scanf("%s",str);
        get_next();
        f[0]=0;
        for(int i=1;i<=L;++i)
        {
            f[i]=f[nextt[i]]+1;
            sum=(sum+f[i])%Mod;
        }
        printf("%d\n",sum);
    }
    return 0;
}