题面
题解
太长了不想写了→_→
//minamoto
#include<bits/stdc++.h>
#define R register
#define inf 0x3f3f3f3f
#define pb push_back
#define inline __inline__ __attribute__((always_inline))
#define fp(i,a,b) for(R int i=(a),I=(b)+1;i<I;++i)
#define fd(i,a,b) for(R int i=(a),I=(b)-1;i>I;--i)
#define go(u) for(int i=head[u],v=e[i].v;i;i=e[i].nx,v=e[i].v)
template<class T>inline bool cmin(T&a,const T&b){return a>b?a=b,1:0;}
using namespace std;
char buf[1<<21],*p1=buf,*p2=buf;
inline char getc(){return p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++;}
int read(){
R int res,f=1;R char ch;
while((ch=getc())>'9'||ch<'0')(ch=='-')&&(f=-1);
for(res=ch-'0';(ch=getc())>='0'&&ch<='9';res=res*10+ch-'0');
return res*f;
}
char sr[1<<21],z[20];int C=-1,Z=0;
inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;}
void print(R int x){
if(C>1<<20)Ot();if(x<0)sr[++C]='-',x=-x;
while(z[++Z]=x%10+48,x/=10);
while(sr[++C]=z[Z],--Z);sr[++C]='
';
}
const int N=5e5+5,M=1e6+5;
struct eg{int u,v,nx,w;}e[M];int head[N],tot;
inline void add(R int u,R int v,R int w){e[++tot]={u,v,head[u],w},head[u]=tot;}
struct node{
int u,d;
inline node(R int uu,R int dd):u(uu),d(dd){}
inline bool operator <(const node &b)const{return d>b.d;}
};priority_queue<node>q;
int dis[2][N],fa[N];bool vis[N],flag[M];
int n,m,res;
void dij(int *dis,int S){
memset(dis,0x3f,(n+1)<<2);
memset(vis,0,(n+1)<<2);
dis[S]=0,q.push(node(S,0));
while(!q.empty()){
int u=q.top().u;q.pop();if(vis[u])continue;vis[u]=1;
go(u)if(cmin(dis[v],dis[u]+e[i].w))q.push(node(v,dis[v]));
}
}
vector<int>g[N];
void dfs(int u){
for(int i=g[u].size()-1,v;~i;--i){
v=g[u][i];
dis[0][v]+dis[1][v]!=dis[0][n]?fa[v]=fa[u]:fa[v]=v;
dfs(v);
}
}
int main(){
// freopen("testdata.in","r",stdin);
n=read(),m=read(),res=inf;
for(R int i=1,u,v,w;i<=m;++i)u=read(),v=read(),w=read(),add(u,v,w),add(v,u,w);
dij(dis[0],1),dij(dis[1],n);
fp(u,1,n)go(u)if(dis[1][u]==dis[1][v]+e[i].w)g[v].pb(u),flag[i]=1;
fa[n]=n,dfs(n);
for(R int i=1,u,v,w;i<=tot;++i){
u=e[i].u,v=e[i].v,w=e[i].w;
if(!flag[i]&&dis[0][u]<=dis[0][v]&&fa[u]!=fa[v]&&dis[0][u]+w+dis[1][v]!=dis[0][n])
cmin(res,dis[0][u]+w+dis[1][v]);
}
printf("%d
",res==inf?-1:res);
return 0;
}