[Usaco2015 Jan]Grass Cownoisseur 图论 tarjan spfa

先缩点，对于缩点后的DAG，正反跑spfa，枚举每条边进行翻转即可

#include<cstdio>
#include<cstring>
#include<iostream>
#include<queue>
using namespace std;
struct pp{
    struct edge{
        int u,v,w,next;
    }ed[200005];
    int e,head[100005];
    pp(){
        e=1;
        memset(head,0,sizeof head);
    }
    void add(int u,int v,int w){
        ed[e].u=u; ed[e].v=v; ed[e].w=w;
        ed[e].next=head[u]; head[u]=e++;
    }
};
 
pp orgp,newp,newf;
int dfn[100005],low[100005],q[100005];
int top=0,tot=0,id[100005],size[100005];
bool bo[100005];
 
void tarjan(int x){
    dfn[x]=low[x]=++top;
    q[top]=x; bo[x]=1;
    for(int i=orgp.head[x];i;i=orgp.ed[i].next){
        int v=orgp.ed[i].v;
        if(!dfn[v]){
            tarjan(v);
            low[x]=min(low[x],low[v]);
        }
        else if(bo[v])
            low[x]=min(low[x],dfn[v]);
    }
    if(dfn[x]==low[x]){
        int y; tot++;
        do{
            y=q[top--];
            bo[y]=0;
            id[y]=tot;
            size[tot]++;
        }while(y!=x);
    }
}
 
int dis[100005][2];
 
void spfa(pp &ppp,int x,int t){
    memset(bo,0,sizeof bo);
    queue<int > q; q.push(x); 
    dis[x][t]=size[x]; int now,v,w;
    while(!q.empty())
    {
        now=q.front(); q.pop(); bo[now]=0;
        for(int i=ppp.head[now];i;i=ppp.ed[i].next)
        {
            v=ppp.ed[i].v; w=ppp.ed[i].w;
            if(dis[v][t]<dis[now][t]+w){
                dis[v][t]=dis[now][t]+w;
                if(!bo[v]){
                    bo[v]=1;
                    q.push(v);
                }
            }
        }
    }
}
 
int n,m;
 
int main()
{
    //freopen("cown.in","r",stdin);
    //freopen("cown.out","w",stdout);
    scanf("%d%d",&n,&m);
    int u,v;
    for(int i=1;i<=m;i++){
        scanf("%d%d",&u,&v);
        orgp.add(u,v,1);
    }
    for(int i=1;i<=n;i++)
        if(!dfn[i])
            tarjan(i);
    for(int i=1;i<=m;i++){
        if(id[orgp.ed[i].u]!=id[orgp.ed[i].v]){
            newp.add(id[orgp.ed[i].u],id[orgp.ed[i].v],size[id[orgp.ed[i].v]]);
            newf.add(id[orgp.ed[i].v],id[orgp.ed[i].u],size[id[orgp.ed[i].u]]);
        }
    }
    int ans=-0x7fffffff;
    memset(dis,-0x3f,sizeof dis);
    spfa(newp,id[1],0);
    spfa(newf,id[1],1);
    for(int i=1;i<newp.e;i++)
    {
        u=newp.ed[i].u;
        v=newp.ed[i].v;
        ans=max(ans,dis[u][1]+dis[v][0]-size[id[1]]);
    }
    printf("%d
",ans);
}

打的好蠢啊QAQ