我用spfa为什么做不出来

#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=1e5+5;
int n;
vector<pair<int,int> >vc[N];
int dis[N];
int cnt;
//bool vis[N];
void spfa(){
	queue<int>q;
	q.push(1);
	memset(dis,0x7f,sizeof dis);
	//vis[1]=1;
	while (q.empty()){
		int u=q.front();
		q.pop();
		for (int i=1;i<=vc[u].size();i++){
			int v=vc[u][i].first,w=vc[u][i].second;
			if (cnt==n){
				return ;
			}
			if (dis[v]>dis[u]+w){
				dis[v]=dis[u]+w;
				q.push(v);
				cnt++;
			}
		}
	}
}
signed main(){
	cin>>n;
	for (int i=1,u,v,w;i<n;i++){
		cin>>u>>v>>w;
		vc[u].push_back({v,w});
		vc[v].push_back({u,w});
	}
	spfa();
	cout<<dis[n];
	return 0;
}