【动态规划】bzoj1638 [Usaco2007 Mar]Cow Traffic 奶牛交通

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
#define N 5001
typedef long long ll;
#define M 50001
int e,v[M],first[N],next[M];
void AddEdge(int U,int V)
{
	v[++e]=V;
	next[e]=first[U];
	first[U]=e;
}
ll f[N],f2[N];
int ru[N],m,n;
int x[M],y[M];
ll ans;
int main()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;++i)
	  {
	  	scanf("%d%d",&x[i],&y[i]);
	  	++ru[y[i]];
		AddEdge(y[i],x[i]);
	  }
	for(int i=1;i<=n;++i)
	  if(!ru[i])
	    f[i]=1;
	for(int i=1;i<=n;++i)
	  for(int j=first[i];j;j=next[j])
	    f[i]+=f[v[j]];
	e=0;
	memset(first+1,0,sizeof(int)*n);
	for(int i=1;i<=m;++i) AddEdge(x[i],y[i]);
	f2[n]=1;
	for(int i=n;i;--i)
	  for(int j=first[i];j;j=next[j])
	    f2[i]+=f2[v[j]];
	for(int i=1;i<=m;++i) ans=max(ans,f[x[i]]*f2[y[i]]);
	printf("%d\n",ans);
	return 0;
}