1875: [SDOI2009]HH去散步 矩阵乘法

由于不能走来时的边，可以把边转化成点，其余就是裸的矩阵乘法了。

#include<iostream>#include<cstdio>#include<cstring>#define M 125#define P 45989using namespace std;int n,m,t,a,b,cnt,sum;int head[25];int list[M],next[M],from[M];inline int read(){    int a=0,f=1; char c=getchar();    while (c<'0'||c>'9') {if (c=='-') f=-1; c=getchar();}    while (c>='0'&&c<='9') {a=a*10+c-'0'; c=getchar();}    return a*f;}struct Matrix{    int a[M][M];    Matrix()    {        memset(a,0,sizeof(a));    }    friend Matrix operator*(Matrix a,Matrix b)    {        Matrix ans;        for (int i=0;i<=cnt;i++)            for (int j=0;j<=cnt;j++)                for (int k=0;k<=cnt;k++)                    (ans.a[i][j]+=a.a[i][k]*b.a[k][j]%P)%=P;        return ans;    }    friend Matrix operator^(Matrix a,int b)    {        Matrix ans;        for (int i=0;i<=cnt;i++) ans.a[i][i]=1;        for (;b;b>>=1,a=a*a)            if (b&1) ans=ans*a;        return ans;    }};Matrix A,ans;inline void insert(int x,int y){    next[++cnt]=head[x];    head[x]=cnt;    from[cnt]=x;    list[cnt]=y;}int main(){    n=read(); m=read(); t=read(); a=read()+1; b=read()+1;    for (int i=1;i<=m;i++)    {        int u=read()+1,v=read()+1;        insert(u,v); insert(v,u);    }    for (int i=head[a];i;i=next[i])        ans.a[0][i]++;    for (int i=1;i<=cnt;i++)        for (int j=head[list[i]];j;j=next[j])            if (j!=i+((i&1)?1:-1)) A.a[i][j]++;    A=A^(t-1);    ans=ans*A;    for (int i=1;i<=cnt;i++)        if (list[i]==b)            (sum+=ans.a[0][i])%=P;    cout << sum << endl;    return 0;}