[朱刘算法推论 拓扑序DP] BZOJ4011 [HNOI2015]落忆枫音

#include<cstdio>
#include<cstdlib>
#include<algorithm>
#define V G[p].v
#define Mod 1000000007
using namespace std;
typedef long long ll;

inline char nc()
{
static char buf[100000],*p1=buf,*p2=buf;
if (p1==p2) { p2=(p1=buf)+fread(buf,1,100000,stdin); if (p1==p2) return EOF; }
return *p1++;
}

inline void read(int &x)
{
char c=nc(),b=1;
for (;!(c>='0' && c<='9');c=nc()) if (c=='-') b=-1;
for (x=0;c>='0' && c<='9';x=x*10+c-'0',c=nc()); x*=b;
}

ll inv[200005];

inline void Pre(){
inv[1]=1;
for (int i=2;i<=200001;i++)
(inv[i]=(Mod-Mod/i)*inv[Mod%i])%=Mod;
}

struct edge{
int u,v;
int next;
};

edge G[200005];
int head[100005],inum;

inline void add(int u,int v,int p){
G[p].u=u; G[p].v=v; G[p].next=head[u]; head[u]=p;
}

int lst[100005],icnt;
int vst[100005];

inline void dfs(int u)
{
vst[u]=1;
for (int p=head[u];p;p=G[p].next)
if (!vst[V])
dfs(V);
lst[++icnt]=u;
}

int n,m,x,y;
int deg[100005];
ll f[100005];

int main()
{
int _u,_v;
Pre();
freopen("t.in","r",stdin);
freopen("t.out","w",stdout);
read(n); read(m); read(x); read(y);
for (int i=1;i<=m;i++)
read(_u),read(_v),add(_u,_v,++inum),deg[_v]++;
deg[y]++;
ll ans=1;
for (int i=2;i<=n;i++)
(ans*=deg[i])%=Mod;
if (y==1)
return printf("%lld\n",ans),0;
dfs(1);
reverse(lst+1,lst+n+1);
f[y]=ans;
for (int i=1;i<=n;i++)
{
int u=lst[i];
(f[u]*=inv[deg[u]])%=Mod;
for (int p=head[u];p;p=G[p].next)
(f[V]+=f[u])%=Mod;
}
(((ans-=f[x])%=Mod)+=Mod)%=Mod;
printf("%lld\n",ans);
}