hdu 1325 Is It A Tree?

在hdu 1272 的基础上稍加修改就ac了

1272已经判断了无向情况下是否是树形结构，因此我们只需要多判断一下入度为0的点是否只有一个就好了

#include<iostream>
#define maxn 100000+5
using namespace std;
int a,b;
int flag;
int father[maxn];
int sign[maxn];
int r[maxn];
void ready()
{
	for(int i=1;i<maxn;i++) r[i]=0,father[i]=i,sign[i]=0;
}
int dfs(int x)
{
	if(father[x]!=x)
	{
		father[x]=father[father[x]];
	}
	return father[x];
}
void build(int x,int y)
{
	if(x==y) flag=0;
	else
	{
		if(dfs(x)==dfs(y)) flag=0;
		else
		{
			father[dfs(x)]=dfs(y);
		}
	}
	r[y]++;
}

int main()
{
	int casee=1;
	while(cin>>a>>b)
	{
		ready();
		flag=1;
		if(a<0&&b<0){break;}
		if(!a&&!b) {cout<<"Case "<<casee++<<" is a tree."<<endl;continue;}
		build(a,b);sign[a]=sign[b]=1;
		while(cin>>a>>b&&a&&b)
		{
			build(a,b);
			sign[a]=sign[b]=1;
		}
		int k=0,d=0;
		for(int i=1;i<maxn;i++)
		{
			if(sign[i]&&dfs(i)==i) k++;
			if(sign[i]&&r[i]==0) d++;
		}
		if(k==1&&flag&&d==1) cout<<"Case "<<casee++<<" is a tree."<<endl;
		else  cout<<"Case "<<casee++<<" is not a tree."<<endl;
	}
	return 0;
}