基本上是完全不会图论了罢，所以出现脑瘫错误轻喷。

code:

#include<bits/stdc++.h>
using namespace std;
#define int long long
int n,m,u,v,d,i,dis[999999],head[999999],vis[999999],ans[999999],cnt;
struct edge{
    int to,dis,next;
}e[999999];
void add_edge(int u,int v,int d){
    cnt++;
    e[cnt].dis=d;
    e[cnt].to=v;
    e[cnt].next=head[u];
    head[u]=cnt;
}
struct node{
    int dis,pos;
    bool operator <(const node &x)const{
        return x.dis<dis;
    }
};
priority_queue<node> q;
set<int> s[114514];
signed main(){
	cin>>n>>m;
	for(i=1;i<=n;i++) dis[i]=LLONG_MAX;
	for(i=1;i<=m;i++){
		cin>>u>>v>>d;
		add_edge(u,v,d);
		add_edge(v,u,d);
	}
	dis[1]=0;
	q.push((node){0,1});
	while(!q.empty()){
		node tmp=q.top();
		q.pop();
		int x=tmp.pos,d=tmp.dis;
		if(!vis[x]){
			vis[x]=1;
			for(i=head[x];i;i=e[i].next){
				int y=e[i].to,w=0;
				if(s[x].find(d)==s[x].end()) w=1;
                if(dis[y]>dis[x]+w) {
                    dis[y]=dis[x]+w;
                    s[y].clear();
                    s[y].insert(d);
                    q.push((node){dis[y],y});
                }
                else if(dis[y]==dis[x]+w) s[y].insert(d);
			}
		}
	}
	cout<<(dis[n]==LLONG_MAX?-1:dis[n]);
}

WA35