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该题根本不用并查集。树的性质。。附AC代码，，/*水题，根本用不着并查集。。。树的性质：结点的个数==边的个数+1 空树的时候单独判断就行了。。23333333333
*/
#include<cstdio> //顶点的个数==边的个数+1
#include<cstring>
const int MAXN=1000000;
int par[MAXN];
int main()
{
int m,n;
int t=1;
while(scanf("%d%d",&m,&n)&&m>=0&&n>=0){
memset(par,0,sizeof(par));
int e=1;
if((m==0&&n==0)){ //空树
printf("Case %d is a tree.\n",t++);
continue;
}
par[m]=par[n]=1;
while(scanf("%d%d",&m,&n)&&m>0&&n>0){
e++;
par[m]=par[n]=1;
}
int v=0;
for(int i=0;i<MAXN;i++)
v+=par[i];
int flag=(v==e+1);
printf("Case %d ",t++);
if(flag)
puts("is a tree.");
else
puts("is not a tree.");
}
return 0;
}
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