题目问一张无向图能否从1点走到2点再走到3点，且一个点只走一次。
思维定势思维定势。。建图关键在于，源点向2点连边，1点和3点向汇点连边！
另外，题目数据听说有点问题，出现点大于n的数据。。
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>
using namespace std;
#define INF (1<<30)
#define MAXN 666666
#define MAXM 666666 struct Edge{
int v,cap,flow,next;
}edge[MAXM];
int vs,vt,NE,NV;
int head[MAXN]; void addEdge(int u,int v,int cap){
edge[NE].v=v; edge[NE].cap=cap; edge[NE].flow=;
edge[NE].next=head[u]; head[u]=NE++;
edge[NE].v=u; edge[NE].cap=; edge[NE].flow=;
edge[NE].next=head[v]; head[v]=NE++;
} int level[MAXN];
int gap[MAXN];
void bfs(){
memset(level,-,sizeof(level));
memset(gap,,sizeof(gap));
level[vt]=;
gap[level[vt]]++;
queue<int> que;
que.push(vt);
while(!que.empty()){
int u=que.front(); que.pop();
for(int i=head[u]; i!=-; i=edge[i].next){
int v=edge[i].v;
if(level[v]!=-) continue;
level[v]=level[u]+;
gap[level[v]]++;
que.push(v);
}
}
} int pre[MAXN];
int cur[MAXN];
int ISAP(){
bfs();
memset(pre,-,sizeof(pre));
memcpy(cur,head,sizeof(head));
int u=pre[vs]=vs,flow=,aug=INF;
gap[]=NV;
while(level[vs]<NV){
bool flag=false;
for(int &i=cur[u]; i!=-; i=edge[i].next){
int v=edge[i].v;
if(edge[i].cap!=edge[i].flow && level[u]==level[v]+){
flag=true;
pre[v]=u;
u=v;
//aug=(aug==-1?edge[i].cap:min(aug,edge[i].cap));
aug=min(aug,edge[i].cap-edge[i].flow);
if(v==vt){
flow+=aug;
for(u=pre[v]; v!=vs; v=u,u=pre[u]){
edge[cur[u]].flow+=aug;
edge[cur[u]^].flow-=aug;
}
//aug=-1;
aug=INF;
}
break;
}
}
if(flag) continue;
int minlevel=NV;
for(int i=head[u]; i!=-; i=edge[i].next){
int v=edge[i].v;
if(edge[i].cap!=edge[i].flow && level[v]<minlevel){
minlevel=level[v];
cur[u]=i;
}
}
if(--gap[level[u]]==) break;
level[u]=minlevel+;
gap[level[u]]++;
u=pre[u];
}
return flow;
} int main(){
int t,n,m,a,b;
scanf("%d",&t);
while(t--){
scanf("%d%d",&n,&m);
memset(head,-,sizeof(head));
vs=; vt=n<<|; NV=vt+; NE=;
addEdge(vs,,); addEdge(+n,vt,); addEdge(+n,vt,);
for(int i=; i<=n; ++i) addEdge(i,i+n,);
addEdge(,+n,);
while(m--){
scanf("%d%d",&a,&b);
if(a>n || b>n) continue;
addEdge(a+n,b,); addEdge(b+n,a,);
}
if(n<= || ISAP()<) puts("NO");
else puts("YES");
}
return ;
}