【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-2;
    llg sum=1;
    while (b)
    {
        if (b&1) sum*=x,sum%=md;
        x*=x; x%=md;
        b/=2;
    }
    return sum;
}

void init()
{
    llg x,y;
    cin>>n>>m>>s>>t;
    du[t]++,indu[t]++;
    for (llg i=1;i<=m;i++)
    {
        scanf("%lld%lld",&x,&y);
        a[x].push_back(y),du[y]++; indu[y]++;
    }
    ans=1;
    f[t]=1;
    for (llg i=2;i<=n;i++) ans*=indu[i],ans%=md,f[t]*=indu[i],f[t]%=md;
}

void DP()
{
    llg x;
    for (llg i=1;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=0;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==1) {cout<<ans; return 0;}
    DP();
    ans-=f[s]; ans+=md; ans%=md;
    cout<<ans;
    return 0;
}