学步园

#include<iostream>
#include<cstring>
#include<cstdio>
using namespace std;
struct data{
	int from,to,next,w;
}e[1000001];
int n,m,sum=0,head[1001],dis[1001],ans,father[1001],q[1000001];
bool del[1000001],inq[1001];
void insert(int u,int v,int w)
{
	sum++;
	e[sum].from=u;
	e[sum].to=v;
	e[sum].next=head[u];
	e[sum].w=w;
	head[u]=sum;
}
void spfa(int k)
{
	memset(dis,127,sizeof(dis));
	memset(inq,0,sizeof(inq));
	memset(q,0,sizeof(q));
	int t=0,w=1,now;
	q[t]=1;inq[1]=1;dis[1]=0;
	while(t<w){
		int p=head[q[t]];
		now=q[t];t++;
		while(p){
			if(!del[p]&&dis[now]+e[p].w<dis[e[p].to]){
				dis[e[p].to]=dis[now]+e[p].w;
				if(k==1)father[e[p].to]=p;
				if(!inq[e[p].to]){
					inq[e[p].to]=1;
					q[w++]=e[p].to;
				}
			}
			p=e[p].next;
		}
		inq[now]=0;
	}
}
int main()
{
	scanf("%d %d",&n,&m);
	for(int i=1;i<=m;i++){
		int a,b,v;
		scanf("%d %d %d",&a,&b,&v);
		insert(a,b,v);
		insert(b,a,v);
	}
	spfa(1);
	for(int i=n;i!=1;i=e[father[i]].from){
		del[father[i]]=1;spfa(0);del[father[i]]=0;
		ans=max(dis[n],ans);
	}
	printf("%d",ans);
	return 0;
}