洛谷P3244 [HNOI2015]落忆枫音

#include<cstdio>
#include<cstdlib>
#include<algorithm>
#include<cstring>
#include<vector>
#include<queue>
#define MAXN 100000+10
#define MOD 1000000007
#define ll long long
#define pb push_back
using namespace std;
int n,m,s,t;
vector<int> G[MAXN];
int de[MAXN],De[MAXN];
ll inv[MAXN<<1],f[MAXN],ans=1;
void init(){
    scanf("%d%d%d%d",&n,&m,&s,&t);
    for(int i=1;i<=m;i++){
        int x,y;scanf("%d%d",&x,&y);
        G[x].pb(y);
        de[y]++,De[y]++;
    }
    De[t]++;
    int i;
    for(inv[1]=1,i=2;i<=m;i++){
        inv[i]=(MOD-MOD/i)*inv[MOD%i]%MOD;
    }
    for(i=2;i<=n;i++){
        (ans*=De[i])%=MOD;
    }
    f[t]=ans;
}
void TopoSort(){
    queue<int> q;
    for(int i=1;i<=n;i++){
        if(!de[i])q.push(i);
    }
    while(!q.empty()){
        int x=q.front();q.pop();
        (f[x]*=inv[De[x]])%=MOD;
        for(int i=0;i<G[x].size();i++){
            int y=G[x][i];
            (f[y]+=f[x])%=MOD;
            if(!(--de[y])){
                q.push(y);
            }
        }
    }
}
void solve(){
    if(t==1){
        printf("%lld",ans);    
        return;
    }
    TopoSort();
    printf("%lld",(ans-f[s]+MOD)%MOD);
}
int main()
{
//    freopen("data.in","r",stdin);
    init();
    solve();
    return 0;
}