【Noip模拟 20161005】友好城市

#include<bits/stdc++.h>
using namespace std;
#define IL inline
#define int long long
#define RE register int
#define N 100001
int m,n,cnt,tim,ans;
int head[N],dfn[N],low[N];
stack<int>s;
bitset<N>vis;
struct aa{int v,next;}e[N<<1];
IL void addedge(RE u,RE v){
    e[++cnt]=(aa){v,head[u]},head[u]=cnt;
}IL void Tarjan(RE u){
    dfn[u]=low[u]=++tim,vis[u]=1,s.push(u);
    for (RE v,i=head[u];i;i=e[i].next){
        if (!dfn[v=e[i].v]) Tarjan(v),low[u]=min(low[u],low[v]);
        else if (vis[v])    low[u]=min(low[u],dfn[v]); 
    }if (dfn[u]==low[u]){
        RE sum=0;
        do{++sum,vis[s.top()]=0,s.pop();}while(s.top()^u);
        ans=max(ans,sum);
    }
}
signed main(){
//    freopen("friend.in","r",stdin),freopen("friend.out","w",stdout);
    cin>>n>>m;
    for (RE i=1,u,v;i<=m;++i)
        cin>>u>>v,addedge(u,v);
    for (RE i=1;i<=n;++i) if (!dfn[i]) Tarjan(i);
    cout<<ans;
}