Popular Cows

#include <stack>
#include <cstdio>
#include <cstring>
#define  maxn 100005
using namespace std;
stack<int> s;
int dfn[maxn],low[maxn],head[maxn],f[maxn];
bool vis[maxn];
int ind[maxn];
int tot,cnt,sum;
struct edges
{
    int u,v,next;
}edge[maxn];
int n,m;
void addedge(int u,int v)
{
    edge[cnt].u=u;
    edge[cnt].v=v;
    edge[cnt].next=head[u];
    head[u]=cnt++;
}
void Tarjan(int num)
{
    dfn[num]=low[num]=++tot;
    vis[num]=true;
    s.push(num);
    for(int i=head[num];i!=-1;i=edge[i].next)
    {
        int v=edge[i].v;
        if(!dfn[v])
        {
            Tarjan(v);
            low[num]=min(low[num],low[v]);
        }
        else if(vis[num])
        {
            low[num]=min(low[num],dfn[v]);
        }
    }
    if(dfn[num]==low[num])
    {
        sum++;
        while(true)
        {
            int now=s.top();
            s.pop();
            vis[now]=false;
            ind[now]=sum;
            if(now==num) break;
        }
    }
}
int main()
{
    freopen("in.txt","r",stdin);
    while(~scanf("%d%d",&n,&m))
    {
        memset(head, -1, sizeof(head));
        memset(dfn,0, sizeof(dfn));
        memset(low,0, sizeof(low));
        memset(f,0, sizeof(f));
        memset(vis,false, sizeof(vis));
        memset(ind,0, sizeof(ind));
        tot=sum=cnt=0;
        while(!s.empty()) s.pop();
        for (int i = 1; i <= m; i++)
        {
            int u, v;
            scanf("%d%d", &u, &v);
            addedge(u, v);
        }
        for (int i = 1; i <= n; i++)
        {
            if (!dfn[i]) Tarjan(i);
        }
        for (int i = 1; i <= n; i++)
        {
            for (int j = head[i]; j != -1; j = edge[j].next)
            {
                int v = edge[j].v;
                if (ind[i] != ind[v])
                {
                    f[ind[i]]++;
                }
            }
        }
        int tol = 0;
        int index;
        for (int i = 1; i <= sum; i++)
        {
            if (!f[i])
            {
                tol++;
                index = i;
            }
        }
        int ans = 0;
        if (tol == 1)
        {
            for (int i = 1; i <= n; i++)
            {
                if (ind[i] == index) ans++;
            }
            printf("%d\n", ans);
        } else printf("0\n");
    }
    return 0;
}