#include<iostream>
#include<vector>
#include<cstring>
using namespace std;
int t,n,m,u,v,f[551],ans;
vector <int> G[551];
bool dfs(int rt,int co){
    f[rt]=co;
    for(int i=0;i<G[rt].size();i++){
        if(!f[G[rt][i]]) dfs(G[rt][i],3-co);
        else if(f[G[rt][i]]==co) return 0;
    }
    return 1;
}
int main(){
    ios::sync_with_stdio(0);
    cin>>t;
    while(t--){
        memset(f,0,sizeof(f));
        for(int i=1;i<=550;i++) G[i].clear();
        ans=0;
        cin>>n>>m;
        for(int i=1;i<=m;i++){
            cin>>u>>v;
            G[u].push_back(v);
            G[v].push_back(u);
        }
        for(int i=1;i<=n;i++){
            if(!f[i]){
                if(!dfs(i,1)){
                    ans=1;
                    break;
                }
            }
        }
        if(ans) cout<<"NO"<<endl;
        else cout<<"YES"<<endl;
    }
}

https://class.51nod.com/Challenge/Problem.html#problemId=3231