luogu P3119 [USACO15JAN]Grass Cownoisseur G 有向图强连通分量+分层最短路

#include<cstdio>
#include<queue>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
const int N = 400005;
int n, m;
int h[N], e[N], ne[N], idx;
int dfn[N], low[N], timestamp;
int stk[N], top;
bool in_stk[N];
int id[N];
int scc_cnt;
int Size[N];
int dout[N];
int u[N],v[N];
void add(int a, int b)
{
	e[idx] = b, ne[idx] = h[a], h[a] = idx ++ ;
}
void tarjan(int u)
{
	dfn[u] = low[u] = ++ timestamp;
	stk[ ++ top] = u;
	in_stk[u] = true;
	for (int i = h[u]; i != -1; i = ne[i])
	{
		int j = e[i];
		if (!dfn[j])
		{
			tarjan(j);
			low[u] = min(low[u], low[j]);
		}
		else if (in_stk[j])
			//取最小
			low[u] = min(low[u], dfn[j]);
	}
	if (dfn[u] == low[u])
	{
		++ scc_cnt;
		int y;
		do
		{
			y = stk[top -- ];
			in_stk[y] = false;
			id[y] = scc_cnt;
			Size[scc_cnt] ++ ;
		}
		while (y != u);
	}
}
int dis[N];
int vis[N];
queue<int>q;
void spfa()
{
	memset(dis,-100,sizeof dis);
	memset(vis,0,sizeof vis);
	q.push(id[1]);
	dis[id[1]]=0;
	vis[id[1]]=1;
	while(q.size())
	{
		int u=q.front();
		q.pop();
		vis[u]=0;
		for(int i=h[u]; i!=-1; i=ne[i])
		{
			int v=e[i];
			if(dis[v]<dis[u]+Size[u])
			{
				dis[v]=dis[u]+Size[u];
				if(!vis[v])
				{
					q.push(v);
					vis[v]=1;
				}
			}
		}
	}
}
int main()
{
	cin>>n>>m;
	memset(h, -1, sizeof h);
	for(int i=1; i<=m; i++)
	{
		int a, b;
		cin>>a>>b;
		u[i]=a,v[i]=b;
		add(a, b);
	}
	for (int i = 1; i <= n; i ++ )
		if (!dfn[i])
			tarjan(i);
	memset(h,-1,sizeof h);
	for(int i=1; i<=m; i++)
	{
		if(id[u[i]]==id[v[i]])
			continue;
		//正向边 
		add(id[u[i]],id[v[i]]);
		add(id[u[i]]+n,id[v[i]]+n);
		//反向边，只能走一次 
		add(id[v[i]],id[u[i]]+n);
	}
	for(int i=1; i<=scc_cnt; i++)
		Size[i+n]=Size[i];
	spfa();
	cout<<max(dis[id[1]],dis[id[1]+n])<<endl;
	return 0;
}