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\n",ans);

}

打的好蠢啊QAQ