P3119 [USACO15JAN]草鉴定Grass Cownoisseur 缩点 topo或最长路

#include <bits/stdc++.h>
using namespace std;
const int N = 300000;
const int inf = 0x7fffffff;
struct NN{
	int nxt,v;
}edge[N];
int n,m,num,cnt,head[N],ct;
int low[N],dfn[N],book[N],belong[N],sum[N];
void add(int x,int y){
	++ct;
	edge[ct].v = y;
	edge[ct].nxt = head[x];
	head[x] = ct;
}
stack<int> s;
struct topo{
	struct No{
		int v,nxt;
	}edge[N];
	int ct,in[N],head[N],vis[N];
	int f[N];
	void add(int x,int y){
		++ct;
		edge[ct].nxt = head[x];
		edge[ct].v = y;
		head[x] = ct;
		in[y]++;
	}
	void to_Sort(){
		queue<int> q;
		for(int i=1;i<=n;i++) f[i] = -inf;
		//把其他点都设置为负无穷 只有和1所在联通块相连才能更大 
		f[belong[1]] = sum[belong[1]];
		//初始化 防止走不通  f[i] 表示到第 i 块联通分量的最大数目 
		for(int i=1;i<=n;i++){
			if(!in[i])q.push(i);
		}
		while(!q.empty()){
			int v = q.front();	q.pop();	vis[v] = 1;
			for(int i=head[v];i;i=edge[i].nxt){
				f[edge[i].v] = max(f[edge[i].v],f[v]+sum[edge[i].v]);
				in[edge[i].v]--;
				if(!in[edge[i].v]&&!vis[edge[i].v])q.push(edge[i].v),vis[edge[i].v] = 0;
			}
		}
	}
}pos,neg; 
void tarjan(int u){
	dfn[u] = low[u] = ++num;
	s.push(u); book[u]=1;
	for(int v,i=head[u];i;i=edge[i].nxt){
		v = edge[i].v;
		if(!dfn[v]){
			tarjan(v);
			low[u] = min(low[u],low[v]);
		}else if(book[v])low[u] = min(low[u],dfn[v]);
	}
	if(dfn[u]==low[u]){
		int v;
		++cnt;
		do{
		 v = s.top();
		 s.pop();
		 book[v] = 0;
		 belong[v] = cnt;
		 sum[cnt]++;
		}while(u!=v);
	}
}
int main()
{
	scanf("%d %d",&n,&m);
	for(int i=1;i<=m;i++){
		int u,v;
		scanf("%d %d",&u,&v);
		add(u,v);
	}
	for(int i=1;i<=n;i++)
		if(!dfn[i]){
			tarjan(i);
		}	
	for(int i=1;i<=n;i++){
		for(int v,j=head[i];j;j=edge[j].nxt){
			v = edge[j].v;
			if(belong[i]!=belong[v]){
				pos.add(belong[i],belong[v]);
				neg.add(belong[v],belong[i]);
			}
		}
	}
	pos.to_Sort();
	neg.to_Sort(); 
	int ans = sum[belong[1]];//初始化为一个块个数 防止走不通  
	for(int i=1;i<=n;i++){
		for(int v,j=head[i];j;j=edge[j].nxt){
			v = edge[j].v;
			if(pos.f[belong[v]]>0&&neg.f[belong[i]]>0)//两点都必须到达 
			ans = max(ans,pos.f[belong[v]]+neg.f[belong[i]]);
		}	
	}
	printf("%d\n",ans-sum[belong[1]]);
	return 0;
}