记录编号	429411	评测结果	AAAAEWAWEW
题目名称	[USACO Mar07] 奶牛交通	最终得分	50
用户昵称	TARDIS	是否通过	未通过
代码语言	C++	运行时间	0.863 s
提交时间	2017-07-27 09:50:25	内存使用	0.36 MiB

#include<cstdio>
#include<cmath>
#include<algorithm>
#include<iostream>
#include<vector>
#include<queue>
#define XLM
using namespace std;
const int maxn=5010;
struct edge{
	int from,to,time;
	bool operator >(const edge &rhs){
		return rhs.time<time;
	}
	edge(int u,int v,int t):from(u),to(v),time(t){}
};
vector <int> G[maxn];
vector <edge> E;
//int G[maxn][maxn];	
int n,m,from,to,ans=-1;
int inde[maxn];
bool inque[maxn];
void PRINTarray(int *a,int n){
	for (int i=1;i<=n;i++){
		printf("%d ",a[i]);
	}
	puts("");
	return;
}
int main(){
	#ifdef XLM
	freopen("cowtraffic.in","r",stdin);
	freopen("cowtraffic.out","w",stdout);
	#endif
	scanf("%d%d",&n,&m);
	for (int i=1;i<=m;i++){
		scanf("%d%d",&from,&to);
		E.push_back(edge(from,to,0));
		G[from].push_back(E.size()-1);
		inde[to]++;
	}
//	PRINTarray(inde,n);
	queue<int>que;
	for (int i=1;i<=n;i++){
		if (inde[i]==0) que.push(i),inque[i]=1;
	}
	while (!que.empty()){
		int u=que.front();que.pop();inque[u]=0;
		for (int i=0;i<G[u].size();i++){
			edge &e=E[G[u][i]];
//			printf("%d-%d",e.from,e.to);
			if (!inque[e.to]) que.push(e.to);
			e.time++;
			ans=max(ans,e.time);
		}
	}
//	sort(E.begin(),E.end());
	printf("%d",ans);
	return 0;
}