hdu 1325 Is It A Tree? (并查集)

题意：输入一些边之间的关系，判断是不是一颗树

思路：1.不能成环 2.不能有多个根

#include<cstdio>
#include<algorithm>
#include<set>
#include<cstring>
#include<vector>
#include<cstring>
using namespace std;
const int maxn =100001;
int f[maxn];
int r[maxn];
bool flag=0;
int Find(int x)
{
if(f[x]==x)return x;
else return f[x]=Find(f[x]);
}
void Union(int x,int y)
{
int xx=Find(x);
int yy=Find(y);
if(xx!=yy)
{
f[yy]=xx;
}
}
int main()//1.任意两个点有且仅有一条边
{
int t;
int u,v;
int sum;
int cas=0;
// freopen("in.txt","r",stdin);
while(1)
{
sum=0;
flag=0;
memset(r,0,sizeof(r));
while(scanf("%d%d",&u,&v)&&(u+v))
{
if(u<0&&v<0)return 0;
if(f[u]==0)f[u]=u;
if(f[v]==0)f[v]=v;
if(Find(u)==Find(v)||v!=Find(v))flag=1;//可以到达同一个根
if(flag!=1)
Union(u,v);
}
for(int i=1;i<=maxn;i++)
{
if(f[i]!=0)
{
r[Find(f[i])]++;
}
f[i]=0;
}
int t=0;
for(int i=1;i<=maxn;i++)
if(r[i]>0)t++;
if(t>1)flag=1;
if(flag==1)
printf("Case %d is not a tree.\n",++cas);
else
printf("Case %d is a tree.\n",++cas);
}
}