hdu3594 Cactus

仙人掌入门简单题。
先看一篇文档。

#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;
int T, n, uu, vv, hea[20005], cnt, dfn[20005], loo[20005], idx, vis[20005];
bool isok;
struct Edge{
	int too, nxt;
}edge[50005];
void add_edge(int fro, int too){
	edge[++cnt].nxt = hea[fro];
	edge[cnt].too = too;
	hea[fro] = cnt;
}
void dfs(int x){
	if(!isok)	return ;
	dfn[x] = loo[x] = ++idx;
	vis[x] = 1;
	int num=0;
	for(int i=hea[x]; i; i=edge[i].nxt){
		int t=edge[i].too;
		if(vis[t]==2)	isok = false;//1
		else if(!vis[t]){
			dfs(t);
			if(loo[t]>dfn[x])	isok = false;//2
			if(loo[t]<dfn[x]){//3
				num++;
				loo[x] = min(loo[x], loo[t]);
			}
		}
		else{//3
			loo[x] = min(loo[x], dfn[t]);
			num++;
		}
	}
	vis[x] = 2;
	if(num>1)	isok = false;
}
int main(){
	cin>>T;
	while(T--){
		scanf("%d", &n);
		memset(dfn, 0, sizeof(dfn));
		memset(loo, 0, sizeof(loo));
		memset(hea, 0, sizeof(hea));
		memset(vis, 0, sizeof(vis));
		isok = true;
		cnt = idx = 0;
		while(scanf("%d %d", &uu, &vv)!=EOF){
			if(!uu && !vv)	break;
			add_edge(uu+1, vv+1);
		}
		dfs(1);
		if(idx<n)	isok = false;
		printf(isok?"YES\n":"NO\n");
	}
	return 0;
}