【BZOJ】4011: [HNOI2015]落忆枫音

题目链接：http://blog.csdn.net/popoqqq/article/details/45194103

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写代码的时候也没有很清晰....具体看这里吧

 #include<iostream>
 #include<cstdio>
 #include<algorithm>
 #include<vector>
 #include<cstdlib>
 #include<cmath>
 #include<cstring>
 #include<queue>
 using namespace std;
 #define maxn 1001000
 #define md 1000000007
 #define llg long long
 #define yyj(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
 llg n,m,s,y,f[maxn],du[maxn],indu[maxn],ans,t;
 struct node
 {
     llg po;
     bool operator<(const node&a) const{
         return du[a.po]<du[po];
     }
 };

 vector<llg>a[maxn];

 priority_queue<llg>q;

 llg inv(llg x)
 {
     llg b=md-;
     llg sum=;
     while (b)
     {
         if (b&) sum*=x,sum%=md;
         x*=x; x%=md;
         b/=;
     }
     return sum;
 }

 void init()
 {
     llg x,y;
     cin>>n>>m>>s>>t;
     du[t]++,indu[t]++;
     for (llg i=;i<=m;i++)
     {
         scanf("%lld%lld",&x,&y);
         a[x].push_back(y),du[y]++; indu[y]++;
     }
     ans=;
     f[t]=;
     for (llg i=;i<=n;i++) ans*=indu[i],ans%=md,f[t]*=indu[i],f[t]%=md;
 }

 void DP()
 {
     llg x;
     for (llg i=;i<=n;i++) if (!du[i]) q.push(i);
     du[t]--;
     while (!q.empty())
     {
         x=q.top(); q.pop();
         f[x]*=inv(indu[x]); f[x]%=md;
         llg w=a[x].size();
         for (llg i=;i<w;i++)
         {
             llg y=a[x][i];
             f[y]+=f[x],f[y]%=md;
             du[y]--;
             if (!du[y]) q.push(y);
         }
     }
 }

 int main()
 {
     yyj("tree");
     init();
     if (t==) {cout<<ans; return ;}
     DP();
     ans-=f[s]; ans+=md; ans%=md;
     cout<<ans;
     return ;
 }