wa了第一点，求助！！

#include <bits/stdc++.h>
using namespace std;
const int N = 2e3+10,M = 3e3+10;
int T;
int n,m;
struct node{
	int to,next,len;
}a[2*M];
int pre[N],k;
queue<int> q;
bool f[N];
int d[N];
int cnt[N];
void spfa(){
	fill(d+1,d+n+1,0x3f3f3f3f);
	while(!q.empty())q.pop();
	memset(cnt,0,sizeof(cnt));
	memset(f,0,sizeof(f));
	d[1] = 0;
	q.push(1);
	f[1] = true;
	while(!q.empty()){
		int u = q.front();
		for(int i = pre[u];i;i = a[i].next){
			int to = a[i].to;
			if(d[u]+a[i].len<d[to]){
				d[to] = d[u]+a[i].len;
				if(!f[to]){
					if(++cnt[to]>=n){
						printf("YES\n");
						return;
					}
					q.push(to);
					f[to] = true;
				}
			}
		}
		q.pop();
		f[u] = false;
	}
	printf("NO\n");
}
void add(int x,int y,int len){
	k++;
	a[k].to = y;
	a[k].next = pre[x];
	a[k].len = len;
	pre[x] = k;
}
int main(){
	scanf("%d",&T);
	int u,v,w;
	while(T--){
		k = 0;
		memset(pre,0,sizeof(pre));
		scanf("%d%d",&n,&m);
		for(int i = 1;i<=m;i++){
			scanf("%d%d%d",&u,&v,&w);
			add(u,v,w);
			if(w>=0)add(v,u,w);
		}
		spfa();
	}

	return 0;
}