bzoj 1030: [JSOI2007]文本生成器

AC自动机的fail，然后用fail搞搞DP，不懂不懂，，，，啊啊啊啊啊啊

#include<bits/stdc++.h>
#define N 1000005
#define LL long long
#define inf 0x3f3f3f3f
using namespace std;
inline int ra()
{
    int x=0,f=1; char ch=getchar();
    while (ch<'0' || ch>'9') {if (ch=='-') f=-1; ch=getchar();}
    while (ch>='0' && ch<='9') {x=x*10+ch-'0'; ch=getchar();}
    return x*f;
}
const int mod=10007;
int n,m,sz=1,ans1,ans2=1;
int a[6001][27],fail[6001],q[6001],f[101][6001];
char s[101];
bool danger[6001];
void ins()
{
    int L=strlen(s),now=1;
    for (int i=0; i<L; i++)
    {
        int t=s[i]-'A'+1;
        if (a[now][t]) now=a[now][t];
        else now=a[now][t]=++sz;
    }
    danger[now]=1;
}
void ACmatch()
{
    int l=0,r=1; q[0]=1; fail[1]=0;
    while (l<r)
    {
        int x=q[l++];
        for (int i=1; i<=26; i++)
        {
            if (!a[x][i]) continue;
            int k=fail[x];
            while (!a[k][i]) k=fail[k];
            fail[a[x][i]]=a[k][i];
            if (danger[a[k][i]]) danger[a[x][i]]=1;
            q[r++]=a[x][i];
        }
    }
}
void dp(int x)
{
    for (int i=1; i<=sz; i++)
    {
        if (danger[i] || !f[x-1][i]) continue;
        for (int j=1; j<=26; j++)
        {
            int k=i;
            while (!a[k][j]) k=fail[k];
            f[x][a[k][j]]=(f[x][a[k][j]]+f[x-1][i])%mod;
        }
    }
}
int main()
{
    n=ra(); m=ra();
    for (int i=1; i<=26; i++) a[0][i]=1;
    for (int i=1; i<=n; i++)
        scanf("%s",s),ins();
    ACmatch(); f[0][1]=1;
    for (int i=1; i<=m; i++) dp(i);
    for (int i=1; i<=m; i++) ans2=ans2*26%mod;
    for (int i=1; i<=sz; i++)
        if (!danger[i]) ans1=(ans1+f[m][i])%mod;
    printf("%d",(ans2-ans1+mod)%mod);
    return 0;
}