差分约束

#include<bits/stdc++.h>
using namespace std;
inline int read()
{
	int s=0,f=1;
	char ch=getchar();
	while(ch<'0'||'9'<ch) {if(ch=='-') f=-1;ch=getchar();}
	while('0'<=ch&&ch<='9') {s=s*10+ch-'0';ch=getchar();}
	return s*f;
}
int T,n,m;
vector<int>head,to,nxt,val;
void join(int u,int v,int w)
{
	nxt.push_back(head[u]);
	head[u]=to.size();
	to.push_back(v);
	val.push_back(w);
}
#define N 2010
#define M 3010
int dis[N],cnt[N];
bool vis[N];
int q[4*N*N],l=0,r=0;
bool spfa()
{
	l=r=0;
	for(int i=1;i<=n;++i)
	{
		dis[i]=0x3f3f3f3f;
		cnt[i]=0;
		vis[i]=0;
	} 
	vis[1]=1;
	q[l]=1;
	dis[1]=0;
	cnt[1]=1;
	while(l<=r)
	{
		int u=q[l];++l;vis[u]=0;
		for(int i=head[u];i!=-1;i=nxt[i])
		{
			int v=to[i];
			//printf("%d %d %d\n",v,dis[v],dis[u]+val[i]);
			if(dis[v]>dis[u]+val[i])
			{
				dis[v]=dis[u]+val[i];
				
				if(!vis[v])
				{
					++cnt[v];
					if(cnt[v]>n) return 0;
					vis[v]=1;
					q[++r]=v;
				} 
			}
		}
	}
	return 1;
}
int main()
{
	T=read();
	while(T--)
	{
		n=read();m=read();
		
		vector<int>().swap(val);
		vector<int>().swap(nxt);
		vector<int>().swap(to);
		vector<int>().swap(head);
		head.resize(max(2*n+2,2*m+2),-1);
		
		for(int i=1;i<=m;++i)
		{
			int u,v,w;
			u=read();v=read();w=read();
			//printf("cs:%d %d %d\n",u,v,w);
			if(w>=0)
			{
				join(u,v,w);
				join(v,u,w);
			} 
			else 
			{
				join(u,v,w);
			}
		}
		if(spfa()) printf("NO\n");
		else printf("YES\n");
	}
	return 0;
}