#include <cstdio>

 #include <cstring>

 #include <algorithm>

 using namespace std;

 #define rep(i,s,t) for(int i=s;i<=t;i++)

 #define clr(a,b) memset(a,b,sizeof(a))

 const int N = 2e4+;

 const int M = 5e4+;

 int head[N],dfn[N],low[N],belong[N],stk[N];

 bool color[N],instk[N],ok;

 int n,top,tot,index,scnt;

 struct Edge{

     int to,next;

 }edge[M];

 void init(){

     tot=top=index=scnt=;

     clr(head,-);clr(dfn,);clr(low,);clr(belong,);

     clr(stk,);clr(instk,false);clr(color,false);

 }

 void addedge(int u,int v){

     edge[tot].to=v,edge[tot].next=head[u];

     head[u]=tot++;

 }

 void Tarjan(int u){

     dfn[u]=low[u]=++index;

     stk[++top]=u,instk[u]=true;

     int cnt=;

     for(int i=head[u];~i;i=edge[i].next){

         int v = edge[i].to;

         if(color[v])ok=false;   //性质1

         if(!dfn[v]){

             Tarjan(v);

             if(low[v]>dfn[u])ok=false; //性质2

             if(low[v]<dfn[u])cnt++;    //u的子节点中low值小于dfn[u]值得个数

             if(cnt==)ok=false;

             low[u]=min(low[u],low[v]);

         }else if(instk[v]){

             low[u]=min(low[u],dfn[v]);cnt++;

             if(cnt==)ok=false;     //性质3

         }

     }

     if(dfn[u]==low[u]){

         scnt++;

         while(true){

             int v=stk[top--];

             instk[v]=false;

             belong[v]=scnt;

             if(v==u)break;

         }

     }

     color[u]=true;

 }

 int main(){

     int T;scanf("%d",&T);while(T--){

         scanf("%d",&n);

         init();

         int u,v;while(scanf("%d%d",&u,&v),u||v){

             u++,v++;

             addedge(u,v);

         }

         ok=true;

         rep(i,,n) if(!dfn[i]) {

             Tarjan(i);

         }

         printf((scnt==&&ok==true)?"YES\n":"NO\n");

     }

 }