hdu 3594 仙人掌图

思路：利用它的几条性质

#include<set>
#include<map>
#include<cmath>
#include<queue>
#include<cstdio>
#include<vector>
#include<string>
#include<cstdlib>
#include<cstring>
#include<iostream>
#include<algorithm>
#define pb push_back
#define mp make_pair
#define Maxn 20010
#define Maxm 2000010
#define LL __int64
#define Abs(x) ((x)>0?(x):(-x))
#define lson(x) (x<<1)
#define rson(x) (x<<1|1)
#define inf 100000
#define lowbit(x) (x&(-x))
#define clr(x,y) memset(x,y,sizeof(x))
#define Mod 1000000007
using namespace std;
int dfn[Maxn],low[Maxn],id[Maxn],vi[Maxn],head[Maxn],use[Maxn],e,num,lab;
void init()
{
    clr(dfn,0);
    clr(low,0);
    clr(id,0);
    clr(vi,0);
    clr(use,0);
    clr(head,-1);
    e=num=lab=0;
}
struct Edge{
    int u,v,col,f,next;
}edge[Maxm];
void add(int u,int v)
{
    edge[e].u=u,edge[e].v=v,edge[e].f=edge[e].col=0,edge[e].next=head[u],head[u]=e++;
}
int Tarjan(int u)
{
    int i,v;
    dfn[u]=low[u]=++lab;
    vi[u]=1;
    for(i=head[u];i!=-1;i=edge[i].next){
        v=edge[i].v;
        if(use[v]) return 0;
        if(!dfn[v]){
            if(!Tarjan(v))
                return 0;
            if(low[v]>dfn[u]) return 0;
            low[u]=min(low[u],low[v]);
        }
        if(vi[v])
            low[u]=min(low[u],dfn[v]);
    }
    use[u]=1;
    if(low[u]==dfn[u]){
        if(u!=1)
            return 0;
    }
    return 1;
}
int main()
{
    int t,n,i,j,u,v;
    scanf("%d",&t);
    while(t--){
        init();
        scanf("%d",&n);
        while(scanf("%d%d",&u,&v),u|v){
            add(u,v);
        }
        //cout<<"ok"<<endl;
        int f=Tarjan(1);
        for(i=0;i<n;i++){
            if(!dfn[i]){
                f=0;
                break;
            }
        }
        if(f)
            printf("YES
");
        else
            printf("NO
");
    }
    return 0;
}