[SDOI2009]Elaxia的路线 最短路+拓扑排序

#include <bits/stdc++.h>
using namespace std;
#define INF 0x7fffffff
const int N = 1550;
int dis[5][N];
int head1[N],head2[N];
int len[N];
int cnt1,cnt2;
int n,m;
struct Edge{
	int w,v,nxt;
};
Edge edge1[N*N],edge2[N*N];
int in[N],book[N];
void add1(int u,int v,int w){
	cnt1++;
	edge1[cnt1].v = v;
	edge1[cnt1].w = w;
	edge1[cnt1].nxt = head1[u];
	head1[u] = cnt1;
}
void add2(int u,int v,int w){
	cnt2++;
	edge2[cnt2].v = v;
	edge2[cnt2].w = w;
	edge2[cnt2].nxt = head2[u];
	head2[u] = cnt2;
	in[v]++;
}
void spfa(int id,int s){
	memset(book,0,sizeof(book));
	queue<int> q;while(!q.empty())q.pop();
	for(int i=1;i<=n;i++)
	dis[id][i] = INF;
	dis[id][s]=0; book[s]=1; q.push(s);
	while(!q.empty()){
		int u=q.front(); book[u]=0; q.pop();
		for(int w,v,i=head1[u];i;i=edge1[i].nxt){
			v = edge1[i].v; w = edge1[i].w+dis[id][u];
			if(dis[id][v]>w){
				dis[id][v]=w;
				if(!book[v]) book[v]=1,q.push(v);
			}
		}
	}	
}
void topo(){
	queue<int> q;while(!q.empty())q.pop();
	for(int i=1;i<=n;i++)if(!in[i])q.push(i);
	while(!q.empty()){
		int u = q.front(); q.pop();
		for(int v,w,i=head2[u];i;i=edge2[i].nxt){
			v = edge2[i].v; w = edge2[i].w + len[u];
			len[v] = max(len[v],w);
			in[v]--;
			if(!in[v])q.push(v);
		}
	}
}
int main()
{
	 scanf("%d %d",&n,&m);
	int x1,y1,x2,y2;
	scanf("%d %d %d %d",&x1,&y1,&x2,&y2);
	for(int i=1;i<=m;i++){
		int u,v,w;
		scanf("%d %d %d",&u,&v,&w);
		add1(u,v,w);
		add1(v,u,w);
	}
	 
	spfa(1,x1); spfa(2,y1); 
	spfa(3,x2); 
	spfa(4,y2);
//	printf("%d",dis[3][4]);
//	printf("%dn",dis[3][8]);
	for(int i=1;i<=n;i++)
	{
		for(int j=head1[i];j;j=edge1[j].nxt)
		{	int v = edge1[j].v,w = edge1[j].w;
			if(dis[1][i]+dis[2][v]+w==dis[1][y1]){
				if(dis[3][i]+dis[4][v]+w==dis[3][y2])
				add2(i,v,w);
			}	
		}
	}
	topo();
	int ans = 0;
	for(int i=1;i<=n;i++)ans=max(ans,len[i]);
	//printf("%dn",ans);
	memset(len,0,sizeof(len));
	memset(in,0,sizeof(in));
	for(int i=1;i<=n;i++)
	{
		for(int j=head1[i];j;j=edge1[j].nxt)
		{	int v = edge1[j].v,w = edge1[j].w;
			if(dis[1][i]+dis[2][v]+w==dis[1][y1]){
				if(dis[3][v]+dis[4][i]+w==dis[3][y2])
				add2(i,v,w);
			}	
		}
	}
	topo();	
	for(int i=1;i<=n;i++)ans=max(ans,len[i]);
	printf("%dn",ans);
	return 0;
 }